Nathan Birrell

Accept any property in a Typescript interface (for use on component props)

May 09, 2018

Sometimes we have core React componentry that passes all props given to it, down to a base DOM UI element. For example, a button element, whilst it may accept a few known props (ie: variations on a button, type={'primary'}) it probably also passes any other unknown props down to the base UI element (ie: aria-label).

To achieve this in Typescript, we can add an indexer to our interface. This is simply a property on the interface that accepts any given property from an object literal. It is still good practice to specify known properties in the interface, especially ones used inside the component itself, not just the props that get spread to the underlying element.

Example on a React component with object destructuring

Combine the indexer with object destructuring and the rest/spread operator, the result is a fairly elegant solution. Take our simple Button component for example:

interface IProps {
  onClick: Function
  type: string,
  // 'Indexer'
  [x: string]: any

class Button extends React.PureComponent<IProps> {
  render() {
    const {
      ...otherProps // Assume any other props are for the base element
    } = this.props

    // do stuff with the type...

    return (
        {...otherProps} // Spread those otherProps

This is a reasonable approach to avoid Typescript complaining about type mismatches for unknown props, while still getting the autocomplete and all on the known props for a component.

Nathan Birrell

Software engineer, primarily focussed on Javascript web application development.
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