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Table of Contents generated with DocToc

Built-in Types

JavaScript defines seven built-in types, which can be broken down into two categories: Primitive Type and Object.

There are six primitive types: null, undefined, boolean, number, string and symbol .

In JavaScript, there are no true integers, all numbers are implemented in double-precision 64-bit binary format IEEE 754. When we use binary floating-point numbers, it will have some side effects. Here is an example of these side effects.

0.1 + 0.2 == 0.3 // false

For the primitive data types, when we use literals to initialize a variable, the variable only has the literals as its value, it doesn’t have a type. It will be converted to the corresponding type only when necessary.

let a = 111 // only literals, not a number
a.toString() // converted to object when necessary

Object is a reference type. We will encouter problems about shallow copy and deep copy when using it.

let a = { name: 'FE' }
let b = a
b.name = 'EF'
console.log(a.name) // EF

Type Conversion

Converting to Boolean

When the condition is judged, other than undefined, null, false, NaN, '', 0, -0, all of the values, including objects, are converted to true.

Objects to Primitive Types

When objects are converted, valueOf and toString will be called, respectively in order. These two methods can also be overridden.

let a = {
    valueOf() {
        return 0
    }
}

Arithmetic Operators

Only for additions, if one of the parameters is a string, the other one will be converted to string as well. For all other operations, as long as one of the parameters is a number, the other one will be converted to a number.

Additions will invoke three types of type conversions: to primitive types, to numbers and to string:

1 + '1' // '11'
2 * '2' // 4
[1, 2] + [2, 1] // '1,22,1'
// [1, 2].toString() -> '1,2'
// [2, 1].toString() -> '2,1'
// '1,2' + '2,1' = '1,22,1'

Note the expression 'a' + + 'b' for addition:

'a' + + 'b' // -> "aNaN"
// since + 'b' -> NaN
// You might have seen + '1' -> 1

== operator

toPrimitive in above figure is converting objects to primitive types.

=== is usually recommended to compare values. However, if you would like to check for null value, you can use xx == null.

Let's take a look at an example [] == ![] // -> true. The following process explains why the expression evaluates to true:

// [] converting to true, then take the opposite to false
[] == false
// with #8
[] == ToNumber(false)
[] == 0
// with #10
ToPrimitive([]) == 0
// [].toString() -> ''
'' == 0
// with #6
0 == 0 // -> true

Comparison Operator

  1. If it's an object, toPrimitive is used.
  2. If it's a string, unicode character index is used.

Typeof

typeof can always display the correct type of primitive types, except null:

typeof 1 // 'number'
typeof '1' // 'string'
typeof undefined // 'undefined'
typeof true // 'boolean'
typeof Symbol() // 'symbol'
typeof b // b is not declared,but it still can be displayed as undefined

For object, typeof will always display object, except function:

typeof [] // 'object'
typeof {} // 'object'
typeof console.log // 'function'

As for null, it is always be treated as an object by typeof，although it is a primitive data type, and this is a bug that has been around for a long time.

typeof null // 'object'

Why does this happen? Because the initial version of JS was based on 32-bit systems, which stored type information of variables in the lower bits for performance considerations. Those start with 000 are objects, and all the bits of null are zero, so it is erroneously treated as an object. Although the current code of checking internal types has changed, this bug has been passed down.

We can use Object.prototype.toString.call(xx) if we want to get the correct data type of a variable, and then we can get a string like [object Type]:

let a
// We can also judge `undefined` like this
a === undefined
// But the nonreserved word `undefined` can be re-assigned in a lower version browser
let undefined = 1
// it will go wrong to judge like this
// So we can use the following method, with less code
// it will always return `undefined`, whatever follows `void `
a === void 0

New

  1. Create a new object
  2. Chained to prototype
  3. Bind this
  4. Return a new object

The above four steps will happen in the process of calling new. We can also try to implement new by ourselves:

function create() {
  // Create an empty object
  let obj = new Object()
  // Get the constructor
  let Ctor = [].shift.call(arguments)
  // Chained to prototype
  obj.__proto__ = Ctor.prototype
  // Bind this, Execute the constructor
  let result = Con.apply(obj, arguments)
  // Make sure the new one is an object
  return typeof result === 'object'? result : obj
}

Instances of object are all created with new, whether it's function Foo() , or let a = { b: 1 } .

It is recommended to create objects using the literal notation (whether it's for performance or readability), since a look-up is needed for Object through the scope chain when creating an object using new Object(), but you don't have this kind of problem when using literals.

function Foo() {}
// Function is a syntactic sugar
// Internally equivalent to new Function()
let a = { b: 1 }
// Inside this literal, `new Object()` is also used

For new, we also need pay attention to the operator precedence:

function Foo() {
    return this;
}
Foo.getName = function () {
    console.log('1');
};
Foo.prototype.getName = function () {
    console.log('2');
};

new Foo.getName();   // -> 1
new Foo().getName(); // -> 2

As you can see from the above image, new Foo() has a higher priority than new Foo, so we can divide the execution order of the above code like this:

new (Foo.getName());
(new Foo()).getName();

For the first function, Foo.getName() is executed first, so the result is 1; As for the latter, it first executes new Foo() to create an instance, then finds the getName function on Foo via the prototype chain, so the result is 2.

This

This, a concept that is confusing to many peole, is actually not difficult to understand as long as you remember the following rules:

function foo() {
  console.log(this.a);
}
var a = 1;
foo();

var obj = {
  a: 2,
  foo: foo
};
obj.foo();

// In the above two situations, `this` only depends on the object before calling the function,
// and the second case has higher priority than the first case .

// the following scenario has the highest priority，`this` will only be bound to c，
// and there's no way to change what `this` is bound to .

var c = new foo();
c.a = 3;
console.log(c.a);

// finally, using `call`, `apply`, `bind` to change what `this` is bound to,
// is another scenario where its priority is only second to `new`

Understanding the above several situations, we won’t be confused by this under most circumstances. Next, let’s take a look at this in arrow functions:

function a() {
  return () => {
    return () => {
      console.log(this);
    };
  };
}
console.log(a()()());

Actually, the arrow function does not have this, this in the above function only depends on the first outer function that is not an arrow function. For this case, this is default to window because calling a matches the first condition in the above codes. Also, what this is bound to will not be changed by any codes once this is bound to the context.

Instanceof

The instanceof operator can correctly check the type of objects, because its internal mechanism is to find out if prototype of this type can be found in the prototype chain of the object.

let’s try to implement it:

function instanceof(left, right) {
    // get the `prototype` of the type
    let prototype = right.prototype
    // get the `prototype` of the object
    left = left.__proto__
    // check if the type of the object is equal to the prototype of the type
    while (true) {
        if (left === null)
            return false
        if (prototype === left)
            return true
        left = left.__proto__
    }
}

Scope

Executing JS code would generate execution context, as long as code is not written in a function, it belongs to the global execution context. Code in a function will generate function execution context. There’s also an eval execution context, which basically is not used anymore, so you can think of only two execution contexts.

The [[Scope]] attribute is generated in the first stage of generating execution context, which is a pointer, corresponds to the linked list of the scope, and JS will look up variables through this linked list up to the global context.

Let's look at a common example , var:

b() // call b
console.log(a) // undefined

var a = 'Hello world'

function b() {
    console.log('call b')
}

It’s known that function and variable hoisting is the real reason for the above outputs. The usual explanation for hoisting says that the declarations are ‘moved’ to the top of the code, and there is nothing wrong with that and it’s easy for everyone to understand. But a more accurate explanation should be something like this:

There would be two stages when the execution context is generated. The first stage is the stage of creation(to be specific, the step of generating variable objects), in which the JS interpreter would find out variables and functions that need to be hoisted, and allocate memory for them in advance, then functions would be stored into memory entirely, but variables would only be declared and assigned to undefined, therefore, we can use them in advance in the second stage (the code execution stage).

In the process of hoisting, the same function would overwrite the last function, and functions have higher priority than variables hoisting.

b() // call b second

function b() {
    console.log('call b fist')
}
function b() {
    console.log('call b second')
}
var b = 'Hello world'

Using var is more likely error-prone, thus ES6 introduces a new keyword let. let has an important feature that it can’t be used before declared, which conflicts the common saying that let doesn’t have the ability of hoisting. In fact, let hoists declaration, but is not assigned, because the temporal dead zone.

Closure

The definition of closure is simple: function A returns a function B, and function B can access variables of function A, thus function B is called a closure.

function A() {
  let a = 1
  function B() {
      console.log(a)
  }
  return B
}

Are you wondering why function B can also refer to variables in function A while function A has been popped up from the call stack? Because variables in function A are stored on the heap at this time. The current JS engine can identify which variables need to be stored on the heap and which need to be stored on the stack by escape analysis.

A classic interview question is using closures in loops to solve the problem of using var to define functions:

for ( var i=1; i<=5; i++) {
    setTimeout( function timer() {
        console.log( i );
    }, i*1000 );
)

First of all, all loops will be executed completely because setTimeout is an asynchronous function, and at that time i is 6, so it will print a bunch of 6.

There are three solutions，closure is the first one:

for (var i = 1; i <= 5; i++) {
  (function(j) {
    setTimeout(function timer() {
      console.log(j);
    }, j * 1000);
  })(i);
}

The second one is to make use of the third parameter of setTimeout:

for ( var i=1; i<=5; i++) {
    setTimeout( function timer(j) {
        console.log( j );
    }, i*1000, i);
}

The third is to define i using let:

for ( let i=1; i<=5; i++) {
    setTimeout( function timer() {
        console.log( i );
    }, i*1000 );
}

For let, it will create a block-level scope, which is equivalent to:

{
    // Form block-level scope
  let i = 0
  {
    let ii = i
    setTimeout( function timer() {
        console.log( i );
    }, i*1000 );
  }
  i++
  {
    let ii = i
  }
  i++
  {
    let ii = i
  }
  ...
}

Prototypes

Each function, besides Function.prototype.bind(), has an internal property, denoted as prototype, which is a reference to the prototype.

Each object has an internal property, denoted as __proto__, which is a reference to the prototype of the constructor that creates the object. This property actually refers to [[prototype]], but [[prototype]] is an internal property that we can’t access, so we use __proto__ to access it.

Objects can use __proto__ to look up properties that do not belong to the object, and __proto__ connects objects together to form a prototype chain.

Inheritance

In ES5, we can solve the problems of inheritance by using the following ways:

function Super() {}
Super.prototype.getNumber = function() {
  return 1
}

function Sub() {}
let s = new Sub()
Sub.prototype = Object.create(Super.prototype, {
  constructor: {
    value: Sub,
    enumerable: false,
    writable: true,
    configurable: true
  }
})

The above idea of inheritance implementation is to set the prototype of the child class as the prototype of the parent class.

In ES6, we can easily solve this problem with the class syntax:

class MyDate extends Date {
  test() {
    return this.getTime()
  }
}
let myDate = new MyDate()
myDate.test()

However, ES6 is not compatible with all browsers, so we need to use Babel to compile this code.

If call myDate.test() with compiled code, you’ll be surprised to see that there’s an error:

Because there are restrictions on the low-level of JS, if the instance isn’t constructed by Date, it can’t call functions in Date, which also explains from another aspect that Class inheritance in ES6 is different from the general inheritance in ES5 syntax.

Since the low-level of JS limits that the instance must be constructed by Date , we can try another way to implement inheritance:

function MyData() {

}
MyData.prototype.test = function () {
  return this.getTime()
}
let d = new Date()
Object.setPrototypeOf(d, MyData.prototype)
Object.setPrototypeOf(MyData.prototype, Date.prototype)

The implementation idea of the above inheritance: first create the instance of parent class => change the original __proto__ of the instance, connect it to the prototype of child class => change the __proto__ of child class’s prototype to the prototype of parent class.

The inheritance implement with the above method can perfectly solve the restriction on low-level of JS.

Deep and Shallow Copy

let a = {
    age: 1
}
let b = a
a.age = 2
console.log(b.age) // 2

From the above example, we can see that if you assign an object to a variable, then the values of both will be the same reference, one changes, the other changes accordingly.

Usually, we don't want such problem to appear during development, thus we can use shallow copy to solve this problem.

Shallow copy

Firstly we can solve the problem by Object.assign:

let a = {
    age: 1
}
let b = Object.assign({}, a)
a.age = 2
console.log(b.age) // 1

Certainly, we can use the spread operator (...) to solve the problem:

let a = {
    age: 1
}
let b = {...a}
a.age = 2
console.log(b.age) // 1

Usually, shallow copy can solve most problems, but we need deep copy when encountering the following situation:

let a = {
    age: 1,
    jobs: {
        first: 'FE'
    }
}
let b = {...a}
a.jobs.first = 'native'
console.log(b.jobs.first) // native

The shallow copy only solves the problem of the first layer. If the object contains objects, then it returns to the beginning topic that both values share the same reference. To solve this problem, we need to introduce deep copy.

Deep copy

The problem can usually be solved by JSON.parse(JSON.stringify(object))

let a = {
    age: 1,
    jobs: {
        first: 'FE'
    }
}
let b = JSON.parse(JSON.stringify(a))
a.jobs.first = 'native'
console.log(b.jobs.first) // FE

But this method also has its limits:

  • ignore undefined
  • unable to serialize function
  • unable to resolve circular references in an object
let obj = {
  a: 1,
  b: {
    c: 2,
    d: 3,
  },
}
obj.c = obj.b
obj.e = obj.a
obj.b.c = obj.c
obj.b.d = obj.b
obj.b.e = obj.b.c
let newObj = JSON.parse(JSON.stringify(obj))
console.log(newObj)

If an object is circularly referenced like the above example, you’ll find the method JSON.parse(JSON.stringify(object)) can’t make a deep copy of this object:

When dealing with function or undefined, the object can also not be serialized properly.

let a = {
    age: undefined,
    jobs: function() {},
    name: 'yck'
}
let b = JSON.parse(JSON.stringify(a))
console.log(b) // {name: "yck"}

In above case, you can see that the method ignores function and undefined.

Most often complex data can be serialized, so this method can solve most problems, and as a built-in function, it has the fastest performance when dealing with deep copy. Certainly, you can use the deep copy function of lodash when your data contains the above three cases.

If the object you want to copy contains a built-in type but doesn’t contain a function, you can use MessageChannel

function structuralClone(obj) {
  return new Promise(resolve => {
    const {port1, port2} = new MessageChannel();
    port2.onmessage = ev => resolve(ev.data);
    port1.postMessage(obj);
  });
}

var obj = {a: 1, b: {
    c: b
}}
// pay attention that this method is asynchronous
// it can handle `undefined` and circular reference object
const clone = await structuralClone(obj);

Modularization

With Babel, we can directly use ES6's modularization:

// file a.js
export function a() {}
export function b() {}
// file b.js
export default function() {}

import {a, b} from './a.js'
import XXX from './b.js'

CommonJS

CommonJS is Node's unique feature. Browserify is needed for CommonJS to be used in browsers.

// a.js
module.exports = {
    a: 1
}
// or
exports.a = 1

// b.js
var module = require('./a.js')
module.a // -> log 1

In the code above, module.exports and exports can cause confusions. Let us take a peek at the internal implementations:

var module = require('./a.js')
module.a
// this is actually a wrapper of a function to be executed immediately so that we don't mess up the global variables.
// what's important here is that module is a Node only variable.
module.exports = {
    a: 1
}
// basic implementation
var module = {
  exports: {} // exports is an empty object
}
// This is why exports and module.exports have similar usage.
var exports = module.exports
var load = function (module) {
    // to be exported
    var a = 1
    module.exports = a
    return module.exports
};

Let's then talk about module.exports and exports, which have similar usage, but one cannot assign a value to exports directly. The assignment would be a no-op.

The differences between the modularizations in CommonJS and in ES6 are:

  • The former supports dynamic imports, which is require(${path}/xx.js); the latter doesn't support it yet, but there have been proposals.
  • The former uses synchronous imports. Since it is used on the server end and files are local, it doesn't matter much even if the synchronous imports block the main thread. The latter uses asynchronous imports, because it is used in browsers in which file downloads are needed. Rendering process would be affected much if synchronous import was used.
  • The former copies the values when exporting. Even if the values exported change, the values imported will not change. Therefore, if values shall be updated, another import needs to happen. However, the latter uses realtime bindings, the values imported and exported point to the same memory addresses, so the imported values change along with the exported ones.
  • In execution the latter is compiled to require/exports.

AMD

AMD is brought forward by RequireJS.

// AMD
define(['./a', './b'], function(a, b) {
    a.do()
    b.do()
})
define(function(require, exports, module) {
    var a = require('./a')  
    a.doSomething()   
    var b = require('./b')
    b.doSomething()
})

The differences between call, apply, bind

Firstly, let’s tell the difference between the former two.

Both call and apply are used to change what this refers to. Their role is the same, but the way to pass parameters is different.

In addition to the first parameter, call can accept an argument list, while apply accepts a single array of arguments.

let a = {
  value: 1
}
function getValue(name, age) {
  console.log(name)
  console.log(age)
  console.log(this.value)
}
getValue.call(a, 'yck', '24')
getValue.apply(a, ['yck', '24'])

simulation to implement call and apply

We can consider how to implement them from the following rules:

  • If the first parameter isn’t passed, then the first parameter will default to window;
  • Change what this refers to, which makes new object capable of executing the function. Then let’s think like this: add a function to a new object and then delete it after the execution.
Function.prototype.myCall = function (context) {
  var context = context || window
  // Add an property to the `context`
  // getValue.call(a, 'yck', '24') => a.fn = getValue
  context.fn = this
  // take out the rest parameters of `context`
  var args = [...arguments].slice(1)
  // getValue.call(a, 'yck', '24') => a.fn('yck', '24')
  var result = context.fn(...args)
  // delete fn
  delete context.fn
  return result
}

The above is the main idea of simulating call, and the implementation of apply is similar.

Function.prototype.myApply = function (context) {
  var context = context || window
  context.fn = this

  var result
  // There's a need to determine whether to store the second parameter
  // If the second parameter exists, spread it
  if (arguments[1]) {
    result = context.fn(...arguments[1])
  } else {
    result = context.fn()
  }

  delete context.fn
  return result
}

The role of bind is the same as the other two, except that it returns a function. And we can implement currying with bind

let’s simulate bind:

Function.prototype.myBind = function (context) {
  if (typeof this !== 'function') {
    throw new TypeError('Error')
  }
  var _this = this
  var args = [...arguments].slice(1)
  // return a function
  return function F() {
    // we can use `new F()` because it returns a function, so we need to determine
    if (this instanceof F) {
      return new _this(...args, ...arguments)
    }
    return _this.apply(context, args.concat(...arguments))
  }
}

Promise implementation

Promise is a new syntax introduced by ES6, which resolves the problem of callback hell.

Promise can be seen as a state machine and it's initial state is pending. We can change the state to resolved or rejected by using the resolve and reject functions. Once the state is changed, it cannot be changed again.

The function then returns a Promise instance, which is a new instance instead of the previous one. And that's because the Promise specification states that in addition to the pending state, other states cannot be changed, and multiple calls of function then will be meaningless if the same instance is returned.

For then, it can essentially be seen as flatMap:

// three states
const PENDING = 'pending';
const RESOLVED = 'resolved';
const REJECTED = 'rejected';
// promise accepts a function argument that will execute immediately.
function MyPromise(fn) {
  let _this = this;
  _this.currentState = PENDING;
  _this.value = undefined;
  // To save the callback of `then`，only cached when the state of the promise is pending,
  //  at most one will be cached in every instance
  _this.resolvedCallbacks = [];
  _this.rejectedCallbacks = [];

  _this.resolve = function(value) {
    // execute asynchronously to guarantee the execution order
    setTimeout(() => {
      if (value instanceof MyPromise) {
        // if value is a Promise, execute recursively
        return value.then(_this.resolve, _this.reject)
      }
      if (_this.currentState === PENDING) {
        _this.currentState = RESOLVED;
        _this.value = value;
        _this.resolvedCallbacks.forEach(cb => cb());
      }
    })
  }

  _this.reject = function(reason) {
    // execute asynchronously to guarantee the execution order
    setTimeout(() => {
      if (_this.currentState === PENDING) {
        _this.currentState = REJECTED;
        _this.value = reason;
        _this.rejectedCallbacks.forEach(cb => cb());
      }
    })
  }

  // to solve the following problem
  // `new Promise(() => throw Error('error))`
  try {
    fn(_this.resolve, _this.reject);
  } catch (e) {
    _this.reject(e);
  }
}

MyPromise.prototype.then = function(onResolved, onRejected) {
  const self = this;
  // specification 2.2.7， `then` must return a new promise
  let promise2;
  // specification 2.2, both `onResolved` and `onRejected` are optional arguments
  // it should be ignored if `onResolved` or `onRjected` is not a function,
  // which implements the penetrate pass of it's value
  // `Promise.resolve(4).then().then((value) => console.log(value))`
  onResolved = typeof onResolved === 'function' ? onResolved : v => v;
  onRejected = typeof onRejected === 'function' ? onRejected : r => throw r;

  if (self.currentState === RESOLVED) {
    return (promise2 = new MyPromise((resolve, reject) => {
      // specification 2.2.4, wrap them with `setTimeout`,
      // in order to insure that `onFulfilled` and `onRjected` execute asynchronously
      setTimeout(() => {
        try {
          let x = onResolved(self.value);
          resolutionProcedure(promise2, x, resolve, reject);
        } catch (reason) {
          reject(reason);
        }
      });
    }));
  }

  if (self.currentState === REJECTED) {
    return (promise2 = new MyPromise((resolve, reject) => {
      // execute `onRejected` asynchronously
      setTimeout(() => {
        try {
          let x = onRejected(self.value);
          resolutionProcedure(promise2, x, resolve, reject);
        } catch (reason) {
          reject(reason);
        }
      });
    }))
  }

  if (self.currentState === PENDING) {
    return (promise2 = new MyPromise((resolve, reject) => {
      self.resolvedCallbacks.push(() => {
         // Considering that it may throw error, wrap them with `try/catch`
        try {
          let x = onResolved(self.value);
          resolutionProcedure(promise2, x, resolve, reject);
        } catch (r) {
          reject(r);
        }
      });

      self.rejectedCallbacks.push(() => {
        try {
          let x = onRejected(self.value);
          resolutionProcedure(promise2, x, resolve, reject);
        } catch (r) {
          reject(r);
        }
      })
    }))
  }
}

// specification 2.3
function resolutionProcedure(promise2, x, resolve, reject) {
  // specification 2.3.1，`x` and  `promise2` can't refer to the same object,
  // avoiding the circular references
  if (promise2 === x) {
    return reject(new TypeError('Error'));
  }

  // specification 2.3.2, if `x` is a Promise and the state is `pending`,
  // the promise must remain, If not, it should execute.
  if (x instanceof MyPromise) {
    if (x.currentState === PENDING) {
      // call the function `resolutionProcedure` again to
      // confirm the type of the argument that x resolves
      // If it's a primitive type, it will be resolved again to
      // pass the value to next `then`.
      x.then((value) => {
        resolutionProcedure(promise2, value, resolve, reject);
      }, reject)
    } else {
      x.then(resolve, reject);
    }
    return;
  }

  // specification 2.3.3.3.3
  // if both `reject` and `resolve` are executed, the first successful
  // execution takes precedence, and any further executions are ignored
  let called = false;
  // specification 2.3.3, determine whether `x` is an object or a function
  if (x !== null && (typeof x === 'object' || typeof x === 'function')) {
    // specification 2.3.3.2, if can't get `then`, execute the `reject`
    try {
      // specification 2.3.3.1
      let then = x.then;
      // if `then` is a function, call the `x.then`
      if (typeof then === 'function') {
        // specification 2.3.3.3
        then.call(x, y => {
          if (called) return;
          called = true;
          // specification 2.3.3.3.1
          resolutionProcedure(promise2, y, resolve, reject);
        }, e => {
          if (called) return;
          called = true;
          reject(e);
        });
      } else {
        // specification 2.3.3.4
        resolve(x);
      }
    } catch (e) {
      if (called) return;
      called = true;
      reject(e);
    }
  } else {
    // specification 2.3.4, `x` belongs to primitive data type
    resolve(x);
  }
}

The above codes, which is implemented based on the Promise / A+ specification, can pass the full test of promises-aplus-tests

Generator Implementation

Generator is an added syntactic feature in ES6. Similar to Promise, it can be used for asynchronous programming.

// * means this is a Generator function
// yield within the block can be used to pause the execution
// next can resume execution
function* test() {
  let a = 1 + 2;
  yield 2;
  yield 3;
}
let b = test();
console.log(b.next()); // >  { value: 2, done: false }
console.log(b.next()); // >  { value: 3, done: false }
console.log(b.next()); // >  { value: undefined, done: true }

As we can tell from the above code, a function with a * would have the next function execution. In other words, the execution of the function returns an object. Every call to the next function can resume executing the paused code. A simple implementation of the Generator function is shown below:

// cb is the compiled 'test' function
function generator(cb) {
  return (function() {
    var object = {
      next: 0,
      stop: function() {}
    };

    return {
      next: function() {
        var ret = cb(object);
        if (ret === undefined) return { value: undefined, done: true };
        return {
          value: ret,
          done: false
        };
      }
    };
  })();
}
// After babel's compilation, 'test' function turns into this:
function test() {
  var a;
  return generator(function(_context) {
    while (1) {
      switch ((_context.prev = _context.next)) {
        // yield splits the code into several blocks
        // every 'next' call executes one block of clode
        // and indicates the next block to execute
        case 0:
          a = 1 + 2;
          _context.next = 4;
          return 2;
        case 4:
          _context.next = 6;
          return 3;
        // execution complete
        case 6:
        case "end":
          return _context.stop();
      }
    }
  });
}

Debouncing

Have you ever encountered this problem in your development: how to do a complex computation in a scrolling event or to prevent the "second accidental click" on a button?

These requirements can be achieved with function debouncing. Especially for the first one, if complex computations are carried out in frequent event callbacks, there's a large chance that the page becomes laggy. It's better to combine multiple computations into a single one, and only operate at particular time. Since there are many libraries that implement debouncing, we won't build our own here and will just take underscore's source code to explain debouncing:

/**
 * underscore's debouncing function. When the callback function is called in series, func will only execute when the idel time is larger or equal to `wait`.
 *
 * @param  {function} func        callback function
 * @param  {number}   wait        length of waiting intervals
 * @param  {boolean}  immediate   when set to true, func is executed immediately
 * @return {function}             returns the function to be called by the client
 */
_.debounce = function(func, wait, immediate) {
    var timeout, args, context, timestamp, result;

    var later = function() {
      // compare now to the last timestamp
      var last = _.now() - timestamp;
      // if the current time interval is smaller than the set interval and larger than 0, then reset the timer.
      if (last < wait && last >= 0) {
        timeout = setTimeout(later, wait - last);
      } else {
        // otherwise it's time to execute the callback function
        timeout = null;
        if (!immediate) {
          result = func.apply(context, args);
          if (!timeout) context = args = null;
        }
      }
    };

    return function() {
      context = this;
      args = arguments;
      // obtain the timestamp
      timestamp = _.now();
      // if the timer doesn't exist then execute the function immediately
      var callNow = immediate && !timeout;
      // if the timer doesn't exist then create one
      // else clear the current timer and reset a new timer
      if (timeout) clearTimeout(timeout);
      timeout = setTimeout(later, wait);
      if (callNow) {
        // if the immediate execution is needed, use apply to start the function
        result = func.apply(context, args);
        context = args = null;
      }

      return result;
    };
  };

The complete function implementation is not too difficult. Let's summarize here.

  • For the implementation of protecting against accidental clicks: as long as I start a timer and the timer is there, no matter how you click the button, the callback function won't be executed. Whenever the timer ends and is set to null, another click is allowed.
  • For the implementation of a delayed function execution: every call to the debouncing function will trigger an evaluation of time interval between the current call and the last one. If the interval is less than the required, another timer will be created, and the delay is set to the set interval minus the previous elapsed time. When the time's up, the corresponding callback function is executed.

Throttle

Debounce and Throttle are different in nature. Debounce is to turn multiple executions into one last execution, and Throttle is to turn multiple executions into executions at regular intervals.

// The first two parameters with debounce are the same function
// options: You can pass two properties
// trailing: Last time does not execute
// leading: First time does not execute
// The two properties cannot coexist, otherwise the function cannot be executed
_.throttle = function(func, wait, options) {
    var context, args, result;
    var timeout = null;
    // Previous timestamp
    var previous = 0;
    // Set empty if options is not passed
    if (!options) options = {};
    // Timer callback function
    var later = function() {
        // If you set `leading`, then set `previous` to zero
        // The first `if` statement of the following function is used
        previous = options.leading === false ? 0 : _.now();
        // The first is prevented memory leaks and the second is judged the following timers when setting `timeout` to null
        timeout = null;
        result = func.apply(context, args);
        if (!timeout) context = args = null;
    };
    return function() {
        // Get current timestamp
        var now = _.now();
        // It must be true when it entering firstly
        // If you do not need to execute the function firstly
        // Set the last timestamp to current
        // Then it will be greater than 0 when the remaining time is calculated next
        if (!previous && options.leading === false)
            previous = now;
        var remaining = wait - (now - previous);
        context = this;
        args = arguments;
        // This condition will only be entered if it set `trailing`
        // This condition will be entered firstly if it not set `leading`
        // Another point, you may think that this condition will not be entered if you turn on the timer
        // In fact, it will still enter because the timer delay is not accurate
        // It is very likely that you set 2 seconds, but it needs 2.2 seconds to trigger, then this time will enter this condition
        if (remaining <= 0 || remaining > wait) {
            // Clean up if there exist a timer otherwise it call twice callback
            if (timeout) {
                clearTimeout(timeout);
                timeout = null;
            }
            previous = now;
            result = func.apply(context, args);
            if (!timeout) context = args = null;
        } else if (!timeout && options.trailing !== false) {
            // Judgment whether timer and trailing are set
            // And you can't set leading and trailing at the same time
            timeout = setTimeout(later, remaining);
        }
        return result;
    };
};

Map、FlatMap and Reduce

The effect of Map is to generate a new array, iterate over the original array, take each element out to do some transformation, and then append to the new array.

[1, 2, 3].map((v) => v + 1)
// -> [2, 3, 4]

Map has three parameters, namely the current index element, the index, the original array.

['1','2','3'].map(parseInt)
//  parseInt('1', 0) -> 1
//  parseInt('2', 1) -> NaN
//  parseInt('3', 2) -> NaN

The effect of FlatMap is almost the same with a Map, but the original array will be flatten for multidimensional arrays. You can think of FlatMap as a map and a flatten, which is currently not supported in browsers.

[1, [2], 3].flatMap((v) => v + 1)
// -> [2, 3, 4]

You can achieve this when you want to completely reduce the dimensions of a multidimensional array:

const flattenDeep = (arr) => Array.isArray(arr)
  ? arr.reduce( (a, b) => [...a, ...flattenDeep(b)] , [])
  : [arr]

flattenDeep([1, [[2], [3, [4]], 5]])

The effect of Reduce is to combine the values in the array and get a final value:

function a() {
    console.log(1);
}

function b() {
    console.log(2);
}

[a, b].reduce((a, b) => a(b()))
// -> 2 1

Async and await

async function will return a Promise:

async function test() {
  return "1";
}
console.log(test()); // -> Promise {<resolved>: "1"}

You can think of async as wrapping a function using Promise.resolve().

await can only be used in async functions:

function sleep() {
  return new Promise(resolve => {
    setTimeout(() => {
      console.log('finish')
      resolve("sleep");
    }, 2000);
  });
}
async function test() {
  let value = await sleep();
  console.log("object");
}
test()

The above code will print finish before printing object. Because await waits for the sleep function resolve, even if the synchronization code is followed, it is not executed before the asynchronous code is executed.

The advantage of async and await compared to the direct use of Promise lies in handling the call chain of then, which can produce clear and accurate code. The downside is that misuse of await can cause performance problems because await blocks the code. Perhaps the asynchronous code does not depend on the former, but it still needs to wait for the former to complete, causing the code to lose concurrency.

Let's look at a code that uses await:

var a = 0
var b = async () => {
  a = a + await 10
  console.log('2', a) // -> '2' 10
  a = (await 10) + a
  console.log('3', a) // -> '3' 20
}
b()
a++
console.log('1', a) // -> '1' 1

You may have doubts about the above code, here we explain the principle:

  • First the function b is executed. The variable a is still 0 before execution await 10, Because the Generators are implemented inside await and Generators will keep things in the stack, so at this time a = 0 is saved
  • Because await is an asynchronous operation, it will immediately return a pending state Promise object when it encounter await, and temporarily returning control of the execution code, so that the code outside the function can continue to be executed. console.log('1', a) will be executed first.
  • At this point, the synchronous code is completed and asynchronous code is started. The saved value is used. At this time, a = 10
  • Then comes the usual code execution

Proxy

Proxy is a new feature since ES6. It can be used to define operations in objects:

let p = new Proxy(target, handler);
// `target` represents the object of need to add the proxy
// `handler` customizes operations in the object

Proxy can be handy for implementation of data binding and listening:

let onWatch = (obj, setBind, getLogger) => {
  let handler = {
    get(target, property, receiver) {
      getLogger(target, property)
      return Reflect.get(target, property, receiver);
    },
    set(target, property, value, receiver) {
      setBind(value);
      return Reflect.set(target, property, value);
    }
  };
  return new Proxy(obj, handler);
};

let obj = { a: 1 }
let value
let p = onWatch(obj, (v) => {
  value = v
}, (target, property) => {
  console.log(`Get '${property}' = ${target[property]}`);
})
p.a = 2 // bind `value` to `2`
p.a // -> Get 'a' = 2

Why 0.1 + 0.2 != 0.3

Because JS uses the IEEE 754 double-precision version (64-bit). Every language that uses this standard has this problem.

As we know, computers use binaries to represent decimals, so 0.1 in binary is represented as

// (0011) represents cycle
0.1 = 2^-4 * 1.10011(0011)

How do we come to this binary number? We can try computing it as below:

Binary computations in float numbers are different from those in integers. For multiplications, only the float bits are computed, while the integer bits are used for the binaries for each bit. Then the first bit is used as the most significant bit. Therefore we get 0.1 = 2^-4 * 1.10011(0011).

0.2 is similar. We just need to get rid of the first multiplcation and get 0.2 = 2^-3 * 1.10011(0011).

Back to the double float for IEEE 754 standard. Among the 64 bits, one bit is used for signing, 11 used for integer bits, and the rest 52 bits are floats. Since 0.1 and 0.2 are infinitely cycling binaries, the last bit of the floats needs to indicate whether to round (same as rounding in decimals).

After rounding, 2^-4 * 1.10011...001 becomes 2^-4 * 1.10011(0011 * 12 times)010. After adding these two binaries we get 2^-2 * 1.0011(0011 * 11 times)0100, which is 0.30000000000000004 in decimals.

The native solution to this problem is shown below:

parseFloat((0.1 + 0.2).toFixed(10))

Regular Expressions

Metacharacters

Metacharacter Effect
. matches any character except line terminators: \n, \r, \u2028 or \u2029.
[] matches anything within the brackets. For example, [0-9] can match any number
^ ^9 means matching anything that starts with '9'; [^9] means not matching characters except '9' in between brackets
{1, 2} matches 1 or 2 digit characters
(yck) only matches strings the same as 'yck'
| matches any character before and after |
\ escape character
* matches the preceding expression 0 or more times
+ matches the preceding expression 1 or more times
? the character before '?' is optional

Flags

Flag Effect
i case-insensitive search
g matches globally
m multiline

Character Shorthands

shorthand Effect
\w alphanumeric characters, underline character
\W the opposite of the above
\s any blank character
\S the opposite of the above
\d numbers
\D the opposite of the above
\b start or end of a word
\B the opposite of the above

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