babel-plugin-promote-class-properties

Babel plugin to replace old code taking advantage of ClassProperties feature

Before

class Foo {
  constructor() {
    this.foo = this.foo.bind(this);
    this.bar = this.bar.bind(this);
  }
 
  foo(arg1, arg2) {
    console.log(arg1, arg2)
  }
 
  bar(lol) {
    return lol;
  }
}
 

After

class Foo {
  foo = (arg1, arg2) => {
    console.log(arg1, arg2)
  }
 
  bar = (lol) => {
    return lol;
  }
}
 

Installation

Motivation

Declaring event listeners is a common thing to do on the daily bases of every JS developer. In real life applications we see a lot of this:

class App extends React.Component {
  constructor() {
    this.onLoad = this.onLoad.bind(this);
 
    this.state = {width: 0, height: 0};
  }
 
  onLoad({target: {height, width}}) {
    this.setState({width, height});
  }
 
  render() {
    const {width, height} = this.state;
 
    return (
      <div>
        <div>{width}x{height}</div>
        <img onLoad={this.onLoad} />
      </div>
    );
  }
}

We need to do

this.onLoad = this.onLoad.bind(this);

Because of the way of this context is handled on addEventListener, see The_value_of_this_within_the_handler, from MDN:

If attaching a handler function to an element using addEventListener(), the value of this inside the handler is a reference to the element. It is the same as the value of the currentTarget property of the event argument that is passed to the handler.

Doing that reassignment, we keep the react component instance as this value and we can do operations like setState while still having access to the DOM element.

TODO

• Remove empty constructor
• Handle inline cases => <img onError={this.onImgError.bind(this) />
• Handle => document.body.addEventListener('scroll', this.onScroll.bind(this));