Array.prototype.forEach()

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

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

// : "a"
// : "b"
// : "c"

forEach(callbackFn)
forEach(callbackFn, thisArg)

const ratings = [5, 4, 5];
let sum = 0;

const sumFunction = async (a, b) => a + b;

ratings.forEach(async (rating) => {
  sum = await sumFunction(sum, rating);
});

console.log(sum);
// : 14
// : 0

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

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

// after
items.forEach((item) => {
  copyItems.push(item);
});

const logArrayElements = (element, index /*, array */) => {
  console.log(`a[${index}] = ${element}`);
};

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

class Counter {
  constructor() {
    this.sum = 0;
    this.count = 0;
  }
  add(array) {
    // Only function expressions have their own this bindings.
    array.forEach(function countEntry(entry) {
      this.sum += entry;
      ++this.count;
    }, this);
  }
}

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

const copy = (obj) => {
  const copy = Object.create(Object.getPrototypeOf(obj));
  const propNames = Object.getOwnPropertyNames(obj);
  propNames.forEach((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

const flatten = (arr) => {
  const result = [];
  arr.forEach((item) => {
    if (Array.isArray(item)) {
      result.push(...flatten(item));
    } else {
      result.push(item);
    }
  });
  return result;
};

// 
const nested = [1, 2, 3, [4, 5, [6, 7], 8, 9]];
console.log(flatten(nested)); // [1, 2, 3, 4, 5, 6, 7, 8, 9]

const numbers = [3, -1, 1, 4, 1, 5];
numbers
  .filter((num) => num > 0)
  .forEach((num, idx, arr) => {
    // arr 
    // 
    console.log(arr[idx - 1], num, arr[idx + 1]);
  });
// undefined 3 1
// 3 1 4
// 1 4 1
// 4 1 5
// 1 5 undefined

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

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

console.log({ numCallbackRuns });

// { element: 1 }
// { element: 3 }
// { element: 7 }
// { numCallbackRuns: 3 }

const arrayLike = {
  length: 3,
  0: 2,
  1: 3,
  2: 4,
  3: 5, // length  3  forEach() 
};
Array.prototype.forEach.call(arrayLike, (x) => console.log(x));
// 2
// 3
// 4