@babel/runtime-corejs2

@babel/runtime-corejs2 is a library that contain's Babel modular runtime helpers as well as version 2 of core-js.

Installation​

npm install --save @babel/runtime-corejs2

yarn add @babel/runtime-corejs2

pnpm add @babel/runtime-corejs2

bun add @babel/runtime-corejs2

See also: @babel/runtime.

Usage​

This is meant to be used as a runtime dependency along with the Babel plugin @babel/plugin-transform-runtime. Please check out the documentation in that package for usage.

Why​

Sometimes Babel may inject some code in the output that is the same and thus can be potentially re-used.

For example, with the class transform (without loose mode):

classA{}

turns into:

function_classCallCheck(instance,Constructor){
//...
}

varCircle=functionCircle(){
_classCallCheck(this,Circle);
};

this means every file that contains a class would have the _classCallCheck function repeated each time.

With @babel/plugin-transform-runtime, it would replace the reference to the function to the @babel/runtime-corejs2 version.

var _classCallCheck =require("@babel/runtime-corejs2/helpers/classCallCheck");

varCircle=functionCircle(){
_classCallCheck(this,Circle);
};

@babel/runtime-corejs2 is just the package that contains the implementations of the functions in a modular way.

Difference from @babel/runtime​

This can be used instead of a polyfill for any non-instance methods. It will replace things like Promise or Symbol with the library functions in core-js.

Promise;

turns into:

var _Promise =require("@babel/runtime-corejs2/core-js/promise.js");

This transformation is also applied for Babel's helpers.