Kotlin inspired built in functions for Javascript

kotlin_javascript_functions.js

Object.defineProperty(
  Object.prototype,
  'let',
  {
    value: function (callback) {
      return callback(this);
    },
    enumerable: false,
    writable: false
  }
);

Object.defineProperty(
  Object.prototype,
  'also',
  {
    value: function (callback) {
      callback(this);
      return this;
    }
  }
);

// Emulating Kotlin's run is no problem with a regular function,
// but with an arrow function binding the context is not possible.
// As a fallback we reconstruct a normal function from the arrow functions source code.
// In Chrome the performance penalty is surprisingly little, but in Firefox we're talking one tenth the performance.
Object.defineProperty(
  Object.prototype,
  'run',
  {
    value: function (callback) {
      if (callback.prototype !== undefined) {
        // regular (or native) function
        return callback.apply(this);
      } else {
        // get source, extract body, and reconstruct a regular function
        const source = callback.toString();
        const body = getArrowFunctionBody(source);
        let func = new Function((source.includes(') => {') ? '' : 'return ') + body);
        
        return func.call(this);
      }
    },
    enumerable: false,
    writable: false
  }
);