Support for new.target

The new.target meta-property is new syntax introduced in ES2015. When an instance of a constructor is created via new, the value of new.target is set to be a reference to the constructor function initially used to allocate the instance. If a function is called rather than constructed via new, new.target is set to undefined.

new.target comes in handy when Object.setPrototypeOf or proto needs to be set in a class constructor. One such use case is inheriting from Error in NodeJS v4 and higher.

Example

class CustomError extends Error {
    constructor(message?: string) {
        super(message); // 'Error' breaks prototype chain here
        Object.setPrototypeOf(this, new.target.prototype); // restore prototype chain
    }
}

This results in the generated JS

var CustomError = (function (_super) {
  __extends(CustomError, _super);
  function CustomError() {
    var _newTarget = this.constructor;
    var _this = _super.apply(this, arguments);  // 'Error' breaks prototype chain here
    _this.__proto__ = _newTarget.prototype; // restore prototype chain
    return _this;
  }
  return CustomError;
})(Error);

new.target also comes in handy for writing constructable functions, for example:

function f() {
  if (new.target) { /* called via 'new' */ }
}

Which translates to:

function f() {
  var _newTarget = this && this instanceof f ? this.constructor : void 0;
  if (_newTarget) { /* called via 'new' */ }
}