Array.prototype.forEach()

// Arrow function
forEach((element) => { ... } )
forEach((element, index) => { ... } )
forEach((element, index, array) => { ... } )

// Callback function
forEach(callbackFn)
forEach(callbackFn, thisArg)

// Inline callback function
forEach(function callbackFn(element) { ... })
forEach(function callbackFn(element, index) { ... })
forEach(function callbackFn(element, index, array){ ... })
forEach(function callbackFn(element, index, array) { ... }, thisArg)

// Production steps of ECMA-262, Edition 5, 15.4.4.18
// Reference: https://es5.github.io/#x15.4.4.18

if (!Array.prototype['forEach']) {

  Array.prototype.forEach = function(callback, thisArg) {

    if (this == null) { throw new TypeError('Array.prototype.forEach called on null or undefined'); }

    var T, k;
    // 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;

    // 4. If isCallable(callback) is false, throw a TypeError exception.
    // See: https://es5.github.io/#x9.11
    if (typeof callback !== "function") { throw new TypeError(callback + ' is not a function'); }

    // 5. If thisArg was supplied, let T be thisArg; else let
    // T be undefined.
    if (arguments.length > 1) { 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 (k in O) {

        // 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
  };
}

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

arraySparse.forEach(function(element) {
  console.log(element)
  numCallbackRuns++
})

console.log("numCallbackRuns: ", numCallbackRuns)

// 1
// 3
// 7
// numCallbackRuns: 3
// comment: as you can see the missing value between 3 and 7 didn't invoke callback function.

const items = ['item1', 'item2', 'item3']
const copyItems = []

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

// after
items.forEach(function(item){
  copyItems.push(item)
})

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

// Notice that index 2 is skipped, since there is no item at
// that position in the array...
[2, 5, , 9].forEach(logArrayElements)
// logs:
// a[0] = 2
// a[1] = 5
// a[3] = 9

function Counter() {
  this.sum = 0
  this.count = 0
}
Counter.prototype.add = function(array) {
  array.forEach(function countEntry(entry) {
    this.sum += entry
    ++this.count
  }, this)
}

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

function copy(obj) {
  const copy = Object.create(Object.getPrototypeOf(obj))
  const propNames = Object.getOwnPropertyNames(obj)

  propNames.forEach(function(name) {
    const desc = Object.getOwnPropertyDescriptor(obj, name)
    Object.defineProperty(copy, name, desc)
  })

  return copy
}

const obj1 = { a: 1, b: 2 }
const obj2 = copy(obj1) // obj2 looks like obj1 now

let words = ['one', 'two', 'three', 'four']
words.forEach(function(word) {
  console.log(word)
  if (word === 'two') {
    words.shift() //'one' will delete from array
  }
}) // one // two // four

console.log(words);  //['two', 'three', 'four']

function flatten(arr) {
  const result = []

  arr.forEach(function(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]