Babel Polyfill
There are JavaScript features that can’t be transformed syntactically, usually
because there’s no equivalent functionality implemented — for example,
Promises and generator functions.
Those kinds of features have to be implemented in the browser by a library to
be used in your code, and that’s the work of a polyfill.
The Babel polyfill is composed by core-js and the Regenerator runtime.
Together, they cover all the features in ES2015+.
Advanced Use
As mentioned, Babel can also be used to transform features that haven’t yet
been implemented in the language. A good example of this is the class fields
proposal (currently at TC39 stage 3: candidate). This is particularly popular
among React devs, as it removes the necessity to explicitly bind methods to a
particular component, and also means that a component’s state can be
declared as a class field (potentially eliminating the need for a constructor).
For those of you wanting to use class fields today, you would need to add the
babel-plugin-transform-class-properties as a dev dependency:
npm install --save-dev babel-plugin-transform-class-properties
You’d also update your .babelrc file as follows:
{
"presets": ["env"],
"plugins": ["transform-class-properties"]
}
Now you can write:
class App extends Component {
state = { count: 0 };
incCount = () => {
this.setState(ps => ({ count: ps.count + 1 }));
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