JavaScript Loops

Runs a block of code a set number of times using an initialized counter.

for (let i = 0; i < 5; i++) {
  console.log(i);
}

Repeatedly executes a block of code as long as a specified condition remains true.

let n = 0;
while (n < 3) {
  console.log(n);
  n++;
}

Runs a block of code once before checking if the loop condition is true.

let i = 0;
do {
  console.log(i);
  i++;
} while (i < 3);

Use break to exit a loop immediately and continue to skip the current iteration.

for (let i = 0; i < 5; i++) {
  if (i === 2) continue;
  if (i === 4) break;
  console.log(i);
}

Iterates directly over iterable values like array elements without manual index management.

const nums = [10, 20, 30];
for (const n of nums) {
  console.log(n);
}

Works natively on strings, sets, maps, and other built-in iterable structures.

for (const char of "Hi") {
  console.log(char);
}

Unpacks properties directly within the loop declaration for cleaner object access.

const users = [{ name: "Ada" }, { name: "Bob" }];
for (const { name } of users) {
  console.log(name);
}

Returns index-value pairs to retrieve the loop counter alongside the element.

const items = ["a", "b"];
for (const [index, val] of items.entries()) {
  console.log(index, val);
}

Iterates over the enumerable string property keys of an object.

const user = { name: "Ada", age: 30 };
for (const key in user) {
  console.log(`${key}: ${user[key]}`);
}

Filters out inherited properties to iterate only the object's own direct properties.

for (const key in user) {
  if (user.hasOwnProperty(key)) {
    console.log(key);
  }
}

Converts object properties into key-value arrays for cleaner, modern iteration.

const user = { name: "Ada", age: 30 };
for (const [key, val] of Object.entries(user)) {
  console.log(`${key}: ${val}`);
}

Executes async tasks sequentially, pausing the loop for each promise to resolve.

for (const id of ids) {
  const res = await fetchItem(id);
  console.log(res);
}

Avoid await in forEach; it ignores returned promises and fires them concurrently.

// Broken: will not wait for save to complete
items.forEach(async item => {
  await save(item);
});

Runs all async operations in parallel by mapping items to an array of promises.

const apiCall = id => fetchItem(id);
const results = await Promise.all(
  ids.map(apiCall)
);

Iterates over async iterables, waiting for each value to resolve sequentially.

for await (const chunk of readStream()) {
  console.log(chunk);
}

Runs a loop inside another loop; plain break only escapes the inner loop.

for (let i = 0; i < 2; i++) {
  for (let j = 0; j < 3; j++) {
    console.log(`${i},${j}`);
  }
}

Exits multiple levels of nesting at once by referencing a statement label.

outer: for (let i = 0; i < 3; i++) {
  for (let j = 0; j < 3; j++) {
    if (i === 1) break outer;
  }
}

Cache array length before the loop to avoid querying it on every iteration.

const len = items.length;
for (let i = 0; i < len; i++) {
  console.log(items[i]);
}