troubleshooting.md

Troubleshooting common JavaScript errors

ReferenceError

This error is thrown when a reference made to a variable/item is broken. That is the variable/item doesn’t exist.

For example:

const cat = 'cat';
console.log(cat); // 'cat'
console.log(dog); // ReferenceError: dog is not defined

We have a variable cat initialized to 'cat'. Next, we referred to the cat variable and dog variable. The cat variable exists but dog variable doesn’t.

cat will return 'cat', while dog will throw a reference error because the variable dog can’t be found.

This also occurs if the referenced variable is not in scope:

for (let i = 0; i < 10; i++) {
  const count = 5;
}
console.log(count); //ReferenceError: count is not defined

SyntaxError

This is the most common error we encounter. This error occurs when we type code that the JavaScript engine can't understand.

For example:

let cat h = 'cat'

This code will throw an SyntaxError telling you where the error occured:

SyntaxError: Unexpected identifier 'h'

What is with the lone h? The h being there breaks the code.

TypeError

TypeError is used to indicate an unsuccessful operation when none of the other NativeError objects are an appropriate indication of the failure cause.

TypeError occurs when an operation is performed on a wrong data type. Maybe a boolean is expected but a sting is found.

For example, if we try to convert a number to uppercase like this:

const num = 123;
num.toUpperCase();

This will throw a TypeError:

TypeError: num.toUpperCase is not a function

Another example, if we try to convert an undefined value to uppercase:

const nothing = undefined;
nothing.toUpperCase(;

This will throw a TypeError as well:

TypeError: Cannot read properties of undefined (reading 'toUpperCase')

Changing a value that cannot be changed is another example:

const a = 5;
a = '5';

This will throw a TypeError:

TypeError: Assignment to constant variable

If you try to iterate over a value that is non-iterable, like an object:

const a = { b: 1, c: 2 }; // Object (non-iterable)
for (const x of a) {
  // do stuff
}

This will throw a TypeError:

TypeError: a is not iterable