for...of

for (variable of iterable)
  statement

variable

Receives a value from the sequence on each iteration. May be either a declaration with const, let, or var, or an assignment target (e.g. a previously declared variable, an object property, or a destructuring assignment pattern). Variables declared with var are not local to the loop, i.e. they are in the same scope the for...of loop is in.

iterable

An iterable object. The source of the sequence of values on which the loop operates.

statement

A statement to be executed on every iteration. May reference variable. You can use a block statement to execute multiple statements.

A for...of loop operates on the values sourced from an iterable one by one in sequential order. Each operation of the loop on a value is called an iteration, and the loop is said to iterate over the iterable. Each iteration executes statements that may refer to the current sequence value.

When a for...of loop iterates over an iterable, it first calls the iterable's [Symbol.iterator]() method, which returns an iterator, and then repeatedly calls the resulting iterator's next() method to produce the sequence of values to be assigned to variable.

A for...of loop exits when the iterator has completed (the next() result is an object with done: true). Like other looping statements, you can use control flow statements inside statement:

• break stops statement execution and goes to the first statement after the loop.
• continue stops statement execution and goes to the next iteration of the loop.

If the for...of loop exited early (e.g. a break statement is encountered or an error is thrown), the return() method of the iterator is called to perform any cleanup.

The variable part of for...of accepts anything that can come before the = operator. You can use const to declare the variable as long as it's not reassigned within the loop body (it can change between iterations, because those are two separate variables). Otherwise, you can use let.

const iterable = [10, 20, 30];

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

You can use destructuring to assign multiple local variables, or use a property accessor like for (x.y of iterable) to assign the value to an object property.

However, a special rule forbids using async as the variable name. This is invalid syntax:

let async;
for (async of [1, 2, 3]); // SyntaxError: The left-hand side of a for-of loop may not be 'async'.

This is to avoid syntax ambiguity with the valid code for (async of => {};;), which is a for loop.

Strings are iterated by Unicode code points.

const iterable = "boo";

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

You can iterate over the arguments object to examine all parameters passed into a function.

function foo() {
  for (const value of arguments) {
    console.log(value);
  }
}

foo(1, 2, 3);
// 1
// 2
// 3

The following example adds a read class to paragraphs that are direct descendants of the <article> element by iterating over a NodeList DOM collection.

const articleParagraphs = document.querySelectorAll("article > p");
for (const paragraph of articleParagraphs) {
  paragraph.classList.add("read");
}

Iterating over an object with an [Symbol.iterator]() method that returns a custom iterator:

const iterable = {
  [Symbol.iterator]() {
    let i = 1;
    return {
      next() {
        if (i <= 3) {
          return { value: i++, done: false };
        }
        return { value: undefined, done: true };
      },
    };
  },
};

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

Iterating over an object with an [Symbol.iterator]() generator method:

const iterable = {
  *[Symbol.iterator]() {
    yield 1;
    yield 2;
    yield 3;
  },
};

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

Iterable iterators (iterators with a [Symbol.iterator]() method that returns this) are a fairly common technique to make iterators usable in syntaxes expecting iterables, such as for...of.

let i = 1;

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

for (const value of iterator) {
  console.log(value);
}
// 1
// 2
// 3

Execution of the break statement in the first loop causes it to exit early. The iterator is not finished yet, so the second loop will continue from where the first one stopped at.

const source = [1, 2, 3];

const iterator = source[Symbol.iterator]();

for (const value of iterator) {
  console.log(value);
  if (value === 1) {
    break;
  }
  console.log("This string will not be logged.");
}
// 1

// Another loop using the same iterator
// picks up where the last loop left off.
for (const value of iterator) {
  console.log(value);
}
// 2
// 3

// The iterator is used up.
// This loop will execute no iterations.
for (const value of iterator) {
  console.log(value);
}
// [No output]

Generators implement the return() method, which causes the generator function to early return when the loop exits. This makes generators not reusable between loops.

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

const generator = source();

for (const value of generator) {
  console.log(value);
  if (value === 1) {
    break;
  }
  console.log("This string will not be logged.");
}
// 1

// The generator is used up.
// This loop will execute no iterations.
for (const value of generator) {
  console.log(value);
}
// [No output]

Both for...in and for...of statements iterate over something. The main difference between them is in what they iterate over.

The for...in statement iterates over the enumerable string properties of an object, while the for...of statement iterates over values that the iterable object defines to be iterated over.

The following example shows the difference between a for...of loop and a for...in loop when used with an Array.

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

const iterable = [3, 5, 7];
iterable.foo = "hello";

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

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

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

The object iterable inherits the properties objCustom and arrCustom because it contains both Object.prototype and Array.prototype in its prototype chain.

The for...in loop logs only enumerable properties of the iterable object. It doesn't log array elements 3, 5, 7 or "hello" because those are not properties — they are values. It logs array indexes as well as arrCustom and objCustom, which are actual properties. If you're not sure why these properties are iterated over, there's a more thorough explanation of how array iteration and for...in work.

The second loop is similar to the first one, but it uses Object.hasOwn() to check if the found enumerable property is the object's own, i.e. not inherited. If it is, the property is logged. Properties 0, 1, 2 and foo are logged because they are own properties. Properties arrCustom and objCustom are not logged because they are inherited.

The for...of loop iterates and logs values that iterable, as an array (which is iterable), defines to be iterated over. The object's elements 3, 5, 7 are shown, but none of the object's properties are.

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