Transferring Props

It's a common pattern in React to wrap a component in an abstraction. The outer component exposes a simple property to do something that might have more complex implementation details.

You can use JSX spread attributes to merge the old props with additional values:

<Component {...this.props} more="values" />

If you don't use JSX, you can use any object helper such as ES6 Object.assign or Underscore _.extend:

React.createElement(Component, Object.assign({}, this.props, { more: 'values' }));

The rest of this tutorial explains best practices. It uses JSX and experimental ECMAScript syntax.

Manual Transfer #

Most of the time you should explicitly pass the properties down. This ensures that you only expose a subset of the inner API, one that you know will work.

function FancyCheckbox(props) {
  var fancyClass = props.checked ? 'FancyChecked' : 'FancyUnchecked';
  return (
    <div className={fancyClass} onClick={props.onClick}>
      {props.children}
    </div>
  );
}
ReactDOM.render(
  <FancyCheckbox checked={true} onClick={console.log.bind(console)}>
    Hello world!
  </FancyCheckbox>,
  document.getElementById('example')
);

But what about the name prop? Or the title prop? Or onMouseOver?

Transferring with ... in JSX #

NOTE:

The ... syntax is part of the Object Rest Spread proposal. This proposal is on track to become a standard. See the Rest and Spread Properties ... section below for more details.

Sometimes it's fragile and tedious to pass every property along. In that case you can use destructuring assignment with rest properties to extract a set of unknown properties.

List out all the properties that you would like to consume, followed by ...other.

var { checked, ...other } = props;

This ensures that you pass down all the props EXCEPT the ones you're consuming yourself.

function FancyCheckbox(props) {
  var { checked, ...other } = props;
  var fancyClass = checked ? 'FancyChecked' : 'FancyUnchecked';
  // `other` contains { onClick: console.log } but not the checked property
  return (
    <div {...other} className={fancyClass} />
  );
}
ReactDOM.render(
  <FancyCheckbox checked={true} onClick={console.log.bind(console)}>
    Hello world!
  </FancyCheckbox>,
  document.getElementById('example')
);

NOTE:

In the example above, the checked prop is also a valid DOM attribute. If you didn't use destructuring in this way you might inadvertently pass it along.

Always use the destructuring pattern when transferring unknown other props.

function FancyCheckbox(props) {
  var fancyClass = props.checked ? 'FancyChecked' : 'FancyUnchecked';
  // ANTI-PATTERN: `checked` would be passed down to the inner component
  return (
    <div {...props} className={fancyClass} />
  );
}

Consuming and Transferring the Same Prop #

If your component wants to consume a property but also wants to pass it along, you can repass it explicitly with checked={checked}. This is preferable to passing the full props object since it's easier to refactor and lint.

function FancyCheckbox(props) {
  var { checked, title, ...other } = props;
  var fancyClass = checked ? 'FancyChecked' : 'FancyUnchecked';
  var fancyTitle = checked ? 'X ' + title : 'O ' + title;
  return (
    <label>
      <input {...other}
        checked={checked}
        className={fancyClass}
        type="checkbox"
      />
      {fancyTitle}
    </label>
  );
}

NOTE:

Order matters. By putting the {...other} before your JSX props you ensure that the consumer of your component can't override them. In the example above we have guaranteed that the input will be of type "checkbox".

Rest and Spread Properties ... #

Rest properties allow you to extract the remaining properties from an object into a new object. It excludes every other property listed in the destructuring pattern.

This is an experimental implementation of an ECMAScript proposal.

var { x, y, ...z } = { x: 1, y: 2, a: 3, b: 4 };
x; // 1
y; // 2
z; // { a: 3, b: 4 }

Note:

To transform rest and spread properties using Babel 6, you need to install the es2015 preset, the transform-object-rest-spread plugin and configure them in the .babelrc file.

Transferring with Underscore #

If you don't use JSX, you can use a library to achieve the same pattern. Underscore supports _.omit to filter out properties and _.extend to copy properties onto a new object.

function FancyCheckbox(props) {
  var checked = props.checked;
  var other = _.omit(props, 'checked');
  var fancyClass = checked ? 'FancyChecked' : 'FancyUnchecked';
  return (
    React.DOM.div(_.extend({}, other, { className: fancyClass }))
  );
}