How can I remove an attribute from a Reactjs component's state object

You can set foo to undefined, like so

var Hello = React.createClass({
    getInitialState: function () {
        return {
            foo: 10,
            bar: 10
        }
    },

    handleClick: function () {
        this.setState({ foo: undefined });
    },

    render: function() {
        return (
            <div>
                <div onClick={ this.handleClick.bind(this) }>Remove foo</div>
                <div>Foo { this.state.foo }</div>
                <div>Bar { this.state.bar }</div>
            </div>
        );
    }
});

Example

Update

The previous solution just remove value from foo and key skill exists in state, if you need completely remove key from state, one of possible solution can be setState with one parent key, like so

var Hello = React.createClass({
  getInitialState: function () {
    return {
      data: {
        foo: 10,
        bar: 10
      }
    }
  },
    	
  handleClick: function () {
    const state = {
      data: _.omit(this.state.data, 'foo')
    };
    
    this.setState(state, () => {
      console.log(this.state);
    });
  },
        
  render: function() {
    return (
      <div>
        <div onClick={ this.handleClick }>Remove foo</div>
        <div>Foo { this.state.data.foo }</div>
        <div>Bar { this.state.data.bar }</div>
      </div>
    );
  }
});

ReactDOM.render(<Hello />, document.getElementById('container'))

<script src="https://cdn.jsdelivr.net/lodash/4.17.4/lodash.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react-dom.min.js"></script>
<div id="container"></div>

var Hello = React.createClass({
getInitialState: function () {
    return {
        foo: 10,
        bar: 10
    }
},

handleClick: function () {
    let state = {...this.state};
    delete state.foo;
    this.setState(state);
},

render: function() {
    return (
        <div>
            <div onClick={ this.handleClick.bind(this) }>Remove foo</div>
            <div>Foo { this.state.foo }</div>
            <div>Bar { this.state.bar }</div>
        </div>
    );
}

});

In ReactCompositeComponent.js in the React source on GitHub is a method called _processPendingState, which is the ultimate method which implements merging state from calls to component.setState;

``` _processPendingState: function(props, context) { var inst = this._instance; var queue = this._pendingStateQueue; var replace = this._pendingReplaceState; this._pendingReplaceState = false; this._pendingStateQueue = null;

if (!queue) {
  return inst.state;
}

if (replace && queue.length === 1) {
  return queue[0];
}

var nextState = replace ? queue[0] : inst.state;
var dontMutate = true;
for (var i = replace ? 1 : 0; i < queue.length; i++) {
  var partial = queue[i];
  let partialState = typeof partial === 'function'
    ? partial.call(inst, nextState, props, context)
    : partial;
  if (partialState) {
    if (dontMutate) {
      dontMutate = false;
      nextState = Object.assign({}, nextState, partialState);
    } else {
      Object.assign(nextState, partialState);
    }
  }
}

```

In that code you can see the actual line that implements the merge;

nextState = Object.assign({}, nextState, partialState);

Nowhere in this function is there a call to delete or similar, which means it's not really intended behaviour. Also, completely copying the stat, deleting the property, and calling setState won't work because setState is always a merge, so the deleted property will just be ignored.

Note also that setState does not work immediately, but batches changes, so if you try to clone the entire state object and only make a one-property change, you may wipe over previous calls to setState. As the React document says;

React may batch multiple setState() calls into a single update for performance.

Because this.props and this.state may be updated asynchronously, you should not rely on their values for calculating the next state.

What you could look to do is actually add more info;

this.setState({ xSet: true, x: 'foo' });
this.setState({ xSet: false, x: undefined });

This is ugly, granted, but it gives you the extra piece of info you need to differentiate between a value set to undefined, and a value not set at all. Plus it plays nice with React's internals, transactions, state change batching, and any other horror. Better to take a bit of extra complexity here than try to second-guess Reacts internals, which are full of horrors like transaction reconciliation, managing deprecated features like replaceState, etc