# `.setProps(props[, callback]) => Self`

A method that sets the props of the root component, and re-renders. Useful for when you are
wanting to test how the component behaves over time with changing props. Calling this, for
instance, will call the `componentWillReceiveProps` lifecycle method.

Similar to `setState`, this method accepts a props object and will merge it in with the already
existing props.

NOTE: can only be called on a wrapper instance that is also the root instance.


#### Arguments

1. `props` (`Object`): An object containing new props to merge in with the current state
2. `callback` (`Function` [optional]): If provided, the callback function will be executed once setProps has completed


#### Returns

`ReactWrapper`: Returns itself.



#### Example

```jsx
class Foo extends React.Component {
  render() {
    return (
      <div className={this.props.name}/>
    );
  }
}
```
```jsx
const wrapper = mount(<Foo name="foo" />);
expect(wrapper.find('.foo')).to.have.length(1);
expect(wrapper.find('.bar')).to.have.length(0);
wrapper.setProps({ name: 'bar' });
expect(wrapper.find('.foo')).to.have.length(0);
expect(wrapper.find('.bar')).to.have.length(1);
```

```jsx
const spy = sinon.spy(MyComponent.prototype, 'componentWillReceiveProps');

const wrapper = mount(<MyComponent foo="bar" />);
expect(spy.calledOnce).to.equal(false);
wrapper.setProps({ foo: 'foo' });
expect(spy.calledOnce).to.equal(true);
```


#### Common Gotchas



#### Related Methods

- [`.setState(state) => Self`](setState.md)
- [`.setContext(context) => Self`](setContext.md)