for...in

const object = { a: 1, b: 2, c: 3 };

for (const property in object) {
  console.log(`${property}: ${object[property]}`);
}

// Expected output:
// "a: 1"
// "b: 2"
// "c: 3"

Description

The for...in loop is a loop in JavaScript that iterates over all enumerable string properties of an object.

Parameters

variable, object, statement

for (variable in object)
  statement

Gotcha

Even though the for...in works to loop over an array we should use a traditional for-loop, a forEach() or for...of. As Arrays are just objects in JavaScript it would also loop over properties inherited from the prototype chain. It is also less performant.

References

• mdn

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