Почему вам стоит использовать TypeScript

Array.prototype.forEach = function forEach( callback, thisArg ) {
 
    var T, k;
 
    if ( this == null ) {
      throw new TypeError( "this is null or not defined" );
    }
 
    // 1. Let O be the result of calling ToObject passing the |this| value as the argument.
    var O = Object(this);
 
    // 2. Let lenValue be the result of calling the Get internal method of O with the argument "length".
    // 3. Let len be ToUint32(lenValue).
    var len = O.length >>> 0; // Hack to convert O.length to a UInt32
 
    // 4. If IsCallable(callback) is false, throw a TypeError exception.
    // See: http://es5.github.com/#x9.11
    if ( {}.toString.call(callback) !== "[object Function]" ) {
      throw new TypeError( callback + " is not a function" );
    }
 
    // 5. If thisArg was supplied, let T be thisArg; else let T be undefined.
    if ( thisArg ) {
      T = thisArg;
    }
 
    // 6. Let k be 0
    k = 0;
 
    // 7. Repeat, while k < len
    while( k < len ) {
 
      var kValue;
 
      // a. Let Pk be ToString(k).
      //   This is implicit for LHS operands of the in operator
      // b. Let kPresent be the result of calling the HasProperty internal method of O with argument Pk.
      //   This step can be combined with c
      // c. If kPresent is true, then
      if ( Object.prototype.hasOwnProperty.call(O, k) ) {
 
        // i. Let kValue be the result of calling the Get internal method of O with argument Pk.
        kValue = O[ k ];
 
        // ii. Call the Call internal method of callback with T as the this value and
        // argument list containing kValue, k, and O.
        callback.call( T, kValue, k, O );
      }
      // d. Increase k by 1.
      k++;
    }
    // 8. return undefined
  };