for...of

for (variable of iterable) {
  statement
}

variable

variable iterable. variable const, let var.

iterable

, .

Array

let iterable = [10, 20, 30];

for (let value of iterable) {
  value += 1;
  console.log(value);
}
// 11
// 21
// 31

const let, .

let iterable = [10, 20, 30];

for (const value of iterable) {
  console.log(value);
}
// 10
// 20
// 30

String

let iterable = 'boo';

for (let value of iterable) {
  console.log(value);
}
// "b"
// "o"
// "o"

TypedArray

let iterable = new Uint8Array([0x00, 0xff]);

for (let value of iterable) {
  console.log(value);
}
// 0
// 255

Map

let iterable = new Map([['a', 1], ['b', 2], ['c', 3]]);

for (let entry of iterable) {
  console.log(entry);
}
// ['a', 1]
// ['b', 2]
// ['c', 3]

for (let [key, value] of iterable) {
  console.log(value);
}
// 1
// 2
// 3

Set

let iterable = new Set([1, 1, 2, 2, 3, 3]);

for (let value of iterable) {
  console.log(value);
}
// 1
// 2
// 3

arguments

(function() {
  for (let argument of arguments) {
    console.log(argument);
  }
})(1, 2, 3);

// 1
// 2
// 3

DOM

DOM NodeList: read , :

// :    , 
//  NodeList.prototype[Symbol.iterator]
let articleParagraphs = document.querySelectorAll('article > p');

for (let paragraph of articleParagraphs) {
  paragraph.classList.add('read');
}

for...of break, throw return. .

function* foo(){
  yield 1;
  yield 2;
  yield 3;
};

for (let o of foo()) {
  console.log(o);
  break; //   , 
}

, :

function* fibonacci() {
  // -
  let [prev, curr] = [0, 1];
  for (;;) {
    [prev, curr] = [curr, prev + curr];
    yield curr;
  }
}

for (let n of fibonacci()) {
  //    1000
  if (n > 1000) break;
  console.log(n);
}

, for...of , , break . , .

var gen = (function* () {
  yield 1;
  yield 2;
  yield 3;
})();
for (let o of gen) {
  console.log(o);
  break; //  
}

//    ,    !
for (let o of gen) {
  console.log(o); //   
}

, , iterable:

var iterable = {
  [Symbol.iterator]() {
    return {
      i: 0,
      next() {
        if (this.i < 3) {
          return { value: this.i++, done: false };
        }
        return { value: undefined, done: true };
      }
    };
  }
};

for (var value of iterable) {
  console.log(value);
}
// 0
// 1
// 2

for...of for...in

, for...in for...of . , .

for...in .

for...of , .

for...of for...in Array.

Object.prototype.objCustom = function() {};
Array.prototype.arrCustom = function() {};

let iterable = [3, 5, 7];
iterable.foo = 'hello';

for (let i in iterable) {
  console.log(i); //  0, 1, 2, "foo", "arrCustom", "objCustom"
}

for (let i in iterable) {
  if (iterable.hasOwnProperty(i)) {
    console.log(i); //  0, 1, 2, "foo"
  }
}

for (let i of iterable) {
  console.log(i); //  3, 5, 7
}

.

Object.prototype.objCustom = function() {};
Array.prototype.arrCustom = function() {};

let iterable = [3, 5, 7];
iterable.foo = 'hello';

objCustom Array arrCustom Object Array. iterable objCustom arrCustom - .

for (let i in iterable) {
  console.log(i); //  0, 1, 2, "foo", "arrCustom", "objCustom"
}

iterable, . 3, 5, 7 hello , , . , arrCustom objCustom. , , , array iteration and for...in .

for (let i in iterable) {
  if (iterable.hasOwnProperty(i)) {
    console.log(i); //  0, 1, 2, "foo"
  }
}

, hasOwnProperty() , . . 0, 1, 2 foo ( ). arrCustom objCustom , .

for (let i of iterable) {
  console.log(i); //  3, 5, 7
}

iterable , , .. , 3, 5, 7 .

• Array.prototype.forEach()
• Map.prototype.forEach()
• Object.entries()