javascript - ReactJS bind(this)

Question

I'm following a tutorial to update states in ReactJS. I came across this line in the tutorial this.updateLanguage = this.updateLanguage.bind(this) and I don't understand what is going on. I understand the basics of this and bind, but I've never seen it done like this before. Can someone please explain? The full code:

var React = require('react');

class Popular extends React.Component {
    // constructor to set default state
    constructor (props) {
        super(props);
        this.state = {
            selectLanguage: 'All'
        };
        // I don't understand this line
        this.updateLanguage = this.updateLanguage.bind(this);
    }
    // updates state
    updateLanguage(lang) {
        this.setState(() => {
            return {
                selectLanguage: lang
            }
        });
    }
    render() {
        var languages = ['All', 'JavaScript', 'Ruby', 'Java', 'CSS', 'Python'];
        return (
            <ul className='languages'>
                {languages.map((lang) => {
                    return (
                        // adds listener to current state
                        <li style={lang === this.state.selectLanguage ? {color: '#d0021b'}: null}
                            onClick={this.updateLanguage.bind(null, lang)} 
                            key={lang}>
                            {lang}
                        </li>
                    )
                })}
            </ul>
        )
    }
} 

module.exports = Popular;

Answer 1

DOM callbacks such as click events will set the this context to the DOM element itself, in this case the li. Try removing the part you don't understand and see what happens - you'll see something like this.setState is not defined, because that function isn't defined in the context of the li element (it's basically looking for li.setState).

What bind is doing on that line is ensuring that whenever that function gets called, it will get called with the this context we want, in this case the Popular component - e.g. Popular.setState.

These days it's getting more and more common to see folks just using fat arrow syntax as a shorthand to preserve the this context - e.g. in this case onClick={ () => this.updateLanguage(lang) }.

(note for those concerned about performance: the fat arrow approach is cleaner but somewhat controversial since it's repeatedly declaring the function on every single render. That said, some folks claim there is minimal or no significant performance impact.)