Array.prototype.forEach()

const array1 = ["a", "b", "c"];

array1.forEach((element) => console.log(element));

// Expected output: "a"
// Expected output: "b"
// Expected output: "c"

const arraySparse = [1, 3, , 7];
let numCallbackRuns = 0;

arraySparse.forEach((element) => {
  console.log(element);
  numCallbackRuns++;
});

console.log("numCallbackRuns: ", numCallbackRuns);

// 1
// 3
// 7
// numCallbackRuns: 3
// :       3  7    callback.

const items = ["item1", "item2", "item3"];
const copy = [];

// 
for (let i = 0; i < items.length; i++) {
  copy.push(items[i]);
}

// 
items.forEach(function (item) {
  copy.push(item);
});

function logArrayElements(element, index, array) {
  console.log("a[" + index + "] = " + element);
}

//       2,   ,    
[2, 5, , 9].forEach(logArrayElements);
// :
// a[0] = 2
// a[1] = 5
// a[3] = 9

function Counter() {
  this.sum = 0;
  this.count = 0;
}
Counter.prototype.add = function (array) {
  array.forEach((entry) => {
    this.sum += entry;
    ++this.count;
  }, this);
  // ^---- Note
};

const obj = new Counter();
obj.add([2, 5, 9]);
obj.count;
// 3
obj.sum;
// 16

var THRESHOLD = 12;
var v = [5, 2, 16, 4, 3, 18, 20];
var res;

res = v.every(function (element, index, array) {
  console.log("element:", element);
  if (element >= THRESHOLD) {
    return false;
  }

  return true;
});
console.log("res:", res);
// :
// element: 5
// element: 2
// element: 16
// res: false

res = v.some(function (element, index, array) {
  console.log("element:", element);
  if (element >= THRESHOLD) {
    return true;
  }

  return false;
});
console.log("res:", res);
// :
// element: 5
// element: 2
// element: 16
// res: true

function copy(o) {
  var copy = Object.create(Object.getPrototypeOf(o));
  var propNames = Object.getOwnPropertyNames(o);

  propNames.forEach(function (name) {
    var desc = Object.getOwnPropertyDescriptor(o, name);
    Object.defineProperty(copy, name, desc);
  });

  return copy;
}

var o1 = { a: 1, b: 2 };
var o2 = copy(o1); //  o2  ,   o1

let words = ["one", "two", "three", "four"];
words.forEach((word) => {
  console.log(word);
  if (word === "two") {
    words.shift();
  }
});
// one
// two
// four

function flatten(arr) {
  const result = [];

  arr.forEach((i) => {
    if (Array.isArray(i)) {
      result.push(...flatten(i));
    } else {
      result.push(i);
    }
  });

  return result;
}

// Usage
const nested = [1, 2, 3, [4, 5, [6, 7], 8, 9]];

flatten(nested); // [1, 2, 3, 4, 5, 6, 7, 8, 9]

//   ECMA-262, 5- , 15.4.4.18
//  (en): http://es5.github.io/#x15.4.4.18
//  (ru): http://es5.javascript.ru/x15.4.html#x15.4.4.18
if (!Array.prototype.forEach) {
  Array.prototype.forEach = function (callback, thisArg) {
    var T, k;

    if (this == null) {
      throw new TypeError(" this is null or not defined");
    }

    // 1.  O    ToObject passing the |this| value as the argument.
    var O = Object(this);

    // 2.  lenValue      Get  O   "length".
    // 3.  len  ToUint32(lenValue).
    var len = O.length >>> 0;

    // 4.  IsCallable(callback)  false,   TypeError.
    // : http://es5.github.com/#x9.11
    if (typeof callback !== "function") {
      throw new TypeError(callback + " is not a function");
    }

    // 5.  thisArg ,  T  thisArg;   T  undefined.
    if (arguments.length > 1) {
      T = thisArg;
    }

    // 6.  k  0
    k = 0;

    // 7.  k < len,  
    while (k < len) {
      var kValue;

      // a.  Pk  ToString(k).
      //           in
      // b.  kPresent      HasProperty  O   Pk.
      //          c
      // c.  kPresent  true, 
      if (k in O) {
        // i.  kValue      Get  O   Pk.
        kValue = O[k];

        // ii.    Call  callback   T    this 
        //  ,  kValue, k  O.
        callback.call(T, kValue, k, O);
      }
      // d.  k  1.
      k++;
    }
    // 8.  undefined.
  };
}