Dynamic key in immutability update helper for array

You could use ES6 computed property names, which would look like this with your index variable:

updateValue(index, e) {
  var items = React.addons.update(this.state.items, {
    [index]: {
      amount: {$set: e.target.value}
    }
  })
  this.setState({items})
}

According to this ES6 compatability table, React’s JSX transpiler supports them when its Harmony transforms are enabled, or you could use Babel instead (since react-tools is being deprecated in favour of Babel anyway).

Failing that, you could use a helper function to create an object with a given named property:

function makeProp(prop, value) {
  var obj = {}
  obj[prop] = value
  return obj
}

// ...

updateValue: function(index, e) {
  var items = React.addons.update(this.state.items, makeProp(
    index, {
      amount: {$set: e.target.value}
    }
  ))
  this.setState({items: items})
}