react-form-with-constraints | Simple form validation for React | Validation library

All your code is almost fine, but you need to make some changes to make it work correctly as you expected.

So first, you need to save the last input (number) for the countdown, so let's add lastInput: 0 into your component's state, that state will be filled when your type some number in the input field, so each time you type the lastInput property will change with the value that you set.

Once you have clicked on Stop button, you were doing only this: this.setState({ count: 0 }) but you were forgetting something else, you need to restart the property customNumber too to reset it, so you need to do this: this.setState({ count: 0, customNumber: this.state.lastInput }) where this.state.lastInput has the last value set by the user, that is for avoid to save the input's value into a local variable, so I think it's better to save that value into the component state.

NOTE

If you are using babel in your React component, it will be better using arrow function from ES6, so you can avoid the this.myfunction.bind(this) syntax so you could do it in this way:

taking your code as example it would be like this: