Emotion Styling Guidelines and Best Practices

Emotion Styling Guidelines

DO use styled when you want to include additional (nested) class selectors in your styles DO use styled components when you intend to export a styled component for re-use elsewhere DO use css when you want to amend/merge sets of styles compositionally DO use css when you're making a small, or single-use set of styles for a component DO move your style definitions for the css prop to an external variable when they get long DO prefer tagged template literals (css={css...) over style objects wherever possible for maximum style portability/consistency (note: typescript support may be diminished, but IDE plugins like this make life easy)

DO NOT use styled for small, single-use style tweaks that would be easier to read/review if they were inline DO NOT export incomplete AntD components (make sure all their compound components are exported)

Emotion Tips and Strategies

The first thing to consider when adding styles to an element is how much you think a style might be reusable in other areas of Superset. Always err on the side of reusability here. Nobody wants to chase styling inconsistencies, or try to debug little endless overrides scattered around the codebase. The more we can consolidate, the less will have to be figured out by those who follow. Reduce, reuse, recycle.

When to use css or Styled

In short, either works for just about any use case! And you’ll see them used somewhat interchangeably in the existing codebase. But we need a way to weigh it when we encounter the choice, so here’s one way to think about it:

A good use of styled syntax if you want to re-use a styled component. In other words, if you wanted to export flavors of a component for use, like so:

const StatusThing = styled.div`
  padding: 10px;
  border-radius: 10px;
`;

export const InfoThing = styled(StatusThing)`
  background: blue;
  &::before {
    content: "ℹ️";
  }
`;

export const WarningThing = styled(StatusThing)`
  background: orange;
  &::before {
    content: "⚠️";
  }
`;

export const TerribleThing = styled(StatusThing)`
  background: red;
  &::before {
    content: "🔥";
  }
`;

You can also use styled when you’re building a bigger component, and just want to have some custom bits for internal use in your JSX. For example:

const SeparatorOnlyUsedInThisComponent = styled.hr`
  height: 12px;
  border: 0;
  box-shadow: inset 0 12px 12px -12px rgba(0, 0, 0, 0.5);
`;

function SuperComplicatedComponent(props) {
  return (
    <>
      Daily standup for {user.name}!
      <SeparatorOnlyUsedInThisComponent />
      <h2>Yesterday:</h2>
      // spit out a list of accomplisments
      <SeparatorOnlyUsedInThisComponent />
      <h2>Today:</h2>
      // spit out a list of plans
      <SeparatorOnlyUsedInThisComponent />
      <h2>Tomorrow:</h2>
      // spit out a list of goals
    <>
  );
}

The css prop, in reality, shares all the same styling capabilities as styled but it does have some particular use cases that jump out as sensible. For example if you just want to style one element in your component, you could add the styles inline like so:

function SomeFanciness(props) {
  return (
    <>
      Here's an awesome report card for {user.name}!
      <div 
        css={css`
          box-shadow: 5px 5px 10px #ccc;
          border-radius: 10px;
        `
      }>
        <h2>Yesterday:</h2>
        // ...some stuff
        <h2>Today:</h2>
        // ...some stuff
        <h2>Tomorrow:</h2>
        // ...some stuff
      </div>
    <>
  );
}

You can also define the styles as a variable, external to your JSX. This is handy if the styles get long and you just want it out of the way. This is also handy if you want to apply the same styles to disparate element types, kind of like you might use a CSS class on varied elements. Here’s a trumped up example:

function FakeGlobalNav(props) {
  const menuItemStyles = css`
    display: block;
    border-bottom: 1px solid cadetblue;
    font-family: "Comic Sans", cursive;
  `;

  return (
    <Nav>
      <a css={menuItemStyles} href="#">One link</a>
      <Link css={menuItemStyles} to={url} >Another link</Link>
      <div css={menuItemStyles} onclick={alert('this is not a great example`)} >Another link</Link>
    </Nav>
  );
}

css tips and tricks

Using the theme

css lets you write actual CSS

By default the cssprop uses the object syntax with JS style definitions, like so:

<div css={{
  borderRadius: 10;
  marginTop: 10;
  backgroundColor: `#00FF00`
}}>Howdy</div>

But you can use the css interpolator as well to get away from icky JS styling syntax. Doesn’t this look cleaner?

<div css={css`
  border-radius: 10px;
  margin-top: 10px;
  background-color: `#00FF00`;
`}>Howdy</div>

You might say “whatever… I can read and write JS syntax just fine.” Well, that’s great. But… let’s say you’re migrating in some of our legacy LESS styles… now it’s copy/paste! Or if you want to migrate to or from styled syntax… also copy/paste!

You can combine css definitions with array syntax

You can use multiple groupings of styles with the css interpolator, and combine/override them in array syntax, like so:

function AnotherSillyExampe(props) {
  const shadowedCard = css`
    box-shadow: 2px 2px 4px #999;
    padding: 4px;
  `;
  const infoCard = css`
    background-color: #33f;
    border-radius: 4px;
  `;
  const overrideInfoCard = css`
    background-color: #f33;
  `;
  return (
    <div className="App">
      Combining two classes:
      <div css={[shadowedCard, infoCard]}>Hello</div>
      Combining again, but now with overrides:
      <div css={[shadowedCard, infoCard, overrideInfoCard]}>Hello</div>
    </div>
  );
}

Style variations with props

You can give any component a custom prop, and reference that prop in your component styles, effectively using the prop to turn on a “flavor” of that component

For example, let’s make a styled component that acts as a card. Of course, this could be done with any AntD component, or any component at all. But we’ll do this with a humble div to illustrate the point

const SuperCard = styled.div`
  ${({ theme, cutout }) => `
    padding: ${theme.gridUnit * 2}px;
    border-radius: ${theme.borderRadius}px;
    box-shadow: 10px 5px 10px #ccc ${cutout && `inset`};
    border: 1px solid ${cutout ? transparent : theme.colors.secondary.light3 };
  `
`;

Then just use the component as <SuperCard>Some content</SuperCard> or with the (potentially dynamic) prop: <SuperCard cutout>Some content</SuperCard>

Styled component tips

No need to use theme the hard way:

It’s very tempting (and commonly done) to use the theme prop inline in the template literal like so:

const SomeStyledThing=styled.div`
  padding: ${({ theme }) => theme.gridUnit * 2}px;	
  border-radius: ${({ theme }) => theme.borderRadius}px;
  border: 1px solid ${({ theme }) => theme.colors.secondary.light3};
`;

Instead, you can make things a little easier to read/type by writing it like so:

const SomeStyledThing=styled.div`
  ${({ theme }) => `
    padding: ${theme.gridUnit * 2}px;	
    border-radius: ${theme.borderRadius}px;
    border: 1px solid ${theme.colors.secondary.light3};
  `}
`;

Extend an AntD component with custom styling

As mentioned, you want to keep your styling as close to the root of your component system as possible, to minimize repetitive styling/overrides, and err on the side of reusability. In some cases, that means you’ll want to globally tweak one of our core components to match our design system. In Superset, that’s Ant Design (AntD).

AntD uses a cool trick called compound components. For example, the Menu component also lets you use Menu.Item, Menu.SubMenu, Menu.ItemGroup, and Menu.Divider

The Object.assign trick:

Let's say you want to override an AntD component called Foo, and have Foo.Bar display some custom CSS for the Bar compound component. You can do it effectively like so:

import {
  Foo as AntdFoo,
} from 'antd';

export const StyledBar = styled(AntdFoo.Bar)`
  border-radius: ${({ theme }) => theme.borderRadius}px;
`;

export const Foo = Object.assign(AntdFoo, {
  Bar: StyledBar,
});

You can then import this customized Foo and use Foo.Bar as expected. You should probably save your creation in src/common/components for maximum reusability, and add a Storybook entry so future engineers can view your creation, and designers can better understand how it fits the Superset Design System.