component-design.md

Component Design

For information on how to develop components, see the component development docs.

We use a number of patterns and conventions throughout our components.

Size enums

We use abbreviations to refer to sizes, e.g. xxl, xl, l, m, s, xs, and xxs.

Enums and maps

We define objects which map enums to corresponding values, typically CSS classes. For example, here's how we would define maps for colors and sizes in a fictional MegaMenu component.

// We first define the enum values as a type
type EuiMegaMenuColor = 'primary' | 'secondary' | 'warning' | 'danger';

// Then we define the map for getting the appropriate class for each enum value.
const colorToClassNameMap: { [color in EuiMegaMenuColor]: string } = {
  primary: 'euiMegaMenu--primary',
  secondary: 'euiMegaMenu--secondary',
  warning: 'euiMegaMenu--warning',
  danger: 'euiMegaMenu--danger',
};

// Then we generate the enums themselves by pulling out the keys.
export const COLORS = keysOf(colorToClassNameMap);

This is how we define the prop types using the enums we generated in Typescript:

// We can refer to the enums objects for the prop types.
export type EuiMegaMenuProps = {
  color: EuiMegaMenuColor;
  isDisabled?: boolean;
  /* ... */
};

For the default props we can just specify the enum values we want to use in the constructor and then use the maps to generate the className for the component:

export const EuiMegaMenu: FunctionComponent<EuiMegaMenuProps> = ({
  children,
  className,
  color = 'primary',
  className,
  isDisabled = false,
  ...rest
}) => {
  const classes = classNames(
    'euiMegaMenu',
    colorToClassNameMap[color],
    className,
    {
      'euiMegaMenu--isDisabled': isDisabled,
    },
  );

  /* ... */
}

Pass-through props

To give the consumer as much flexibility as possible we use the destructuring assignment to pull expected props out of the received props and pass ...rest to one of the elements in the render() method. This element is typically the root element, though in rare cases another element makes more sense.

The main benefit behind this practice is that the consumer can specify any of the DOM attributes supported by React, including custom ones with the data- prefix.

In Typescript, it makes sense to then extend the props of that element when declaring the component's type. EUI also provides a shortlist of commonly used props like className, aria-label, and data-test-subj that you should extend as well.

import { HTMLAttributes, FunctionComponent } from 'react';
import { CommonProps } from '../common';

export type EuiMegaMenuProps = HTMLAttributes<HTMLDivElement> &
  CommonProps & {
    color: EuiMegaMenuColor;
    isDisabled?: boolean;
    /* ... */
  };

export const EuiMegaMenu: FunctionComponent<EuiMegaMenuProps> = ({
  children,
  className,
  color = 'primary',
  size,
  className,
  isDisabled = false,
  ...rest
}) => {

  // Anything else specified by the consumer will be applied to the div as a DOM attribute.
  return (
    <div {...rest}>
      {/* ... */}
    </div>
  );
}

Naming props

Enums

String literals should be used wherever possible and prioritized over booleans. This allows for the most extensibility when it comes to adding more features/options in the future. For example, instead of the prop isHorizontal: boolean use layout: 'horizontal' | 'vertical'.

Booleans

Generally, boolean props should have an is prefix, e.g. isPlaceholder or isReadOnly. The exception to this is when the prop matches an existing HTML attribute such as disabled; to avoid confusion the prop name should align with the HTML specification. Mirroring the attributes this way makes the most sense when the component is a thin wrapper around an existing HTML element, e.g. EuiButton -> <button> and EuiRadio -> <input type="radio">.

Event handlers

All event handlers should take the form onEvent and accurately describe when it will be called. e.g. onClick indicates the handler is called when the component is clicked, but if there is more granularity the handler should reflect that with onItemClick, onRowClick, etc.

Common and required props

Try to leverage the children prop wherever possible. This will create a simpler more uniform API throughout our components.

We also require some props to be supported by all components, as reflected in our tests; for example, className. These are easily added via the CommonProps mentioned above.