Understanding this.method.bind(null,...) in ReactJS?

Question

I am working through examples learning ReactJS (Mastering React). While recreating examples, I am clueless with couple of statements and would appreciate some help.

Fiddle: Composing Components

First

...
//Why are we passing null in update.bind(...)?
//Is null here equivalent to 'this'?

<TextBox label='First Name' update={this.update.bind(null,'firstName')}></TextBox>
...

Second

Update method expects a key and a value (method definition below)

...
    update: function(key, value) {
            var newState = {};
            newState[key] = value;
            this.setState(newState);

            //this.setState({[k]:v});
        },
...

However, when it is called with a single parameter, correct key is updated with the right value.

//Aren't we supposed to pass two parameters?  
this.props.update(this.refs.newText.value);

Answer 1

this.update.bind(null,'firstName') that code means - set for this.update this to null, and set first argument as a 'firstName', then when you will call this reference to function - first argument will be 'firstName' and second you can set by yourself., we can simplify code from your example like this

var fn = function (key, value) {
  console.log(key);
  console.log(value);
};

var f = fn.bind(null, 'x');

f('y');