React - Props


props is the only object argument passed from a component to another component.

  • Props are similar to State, but is public and not fully controlled by the component.
  • You can also pass arguments to the whole tree via a context


in React - JSX

<MyComponent name={javascriptVariable} color="Blue"/>

will create two props

  • and props.color



  • must be named from the component's own point of view rather than the context in which it is being used.
  • are read-only. A component must never modify its own props. It must modify it via this.setState() All React components must act like pure functions with respect to their props. See React - State


Default value

Default values are defined by assigning to the special defaultProps property to the component.

class Greeting extends React.Component {
  render() {
    return (
      <h1>Hello, {}</h1>
  • Specifies the default value for the name prop:
Greeting.defaultProps = {
  name: 'Stranger (Default)'
  • And render
  <Greeting />,
<div id="root">
<!-- called the "root" DOM node because everything inside it will be managed by React DOM -->

Type checking

built-in Type checking in React is known as prop-types

Change children type

See How to define the React children type in Typescript?


With the chrome plugin

React Props Chrome Plugin

How to pass the keys of an object as props?

You use the spread operator to destructure the object.


let props = { "x": 10, "y": 20 };
<Component {...props}/>

How to add props and pass parent props to a child?

Example with the spread operator to merge properties.

function Parent(parentProps){
  return <Child  {...{extra: 'value', ...parentProps}}/>

Example how to add a class Name

function Parent(parentProps){
  return <Child  {...{...parentProps, className: 'value '+parentProps.className}}/>

Documentation / Reference

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