Typescript: Iterators (for..in and for..of)

 for..of statements

for..of loops over an iterable object. Here is a simple for..of loop on an array:

let someArray = [1, "string", false];

for (let entry of someArray) {

  console.log(entry); // 1, "string", false

}

for..of vs. for..in statements

Both for..of and for..in statements iterate over lists; the values iterated on are different though, for..in returns a list of keys on the object being iterated, whereas for..of returns a list of values of the numeric properties of the object being iterated.

Here is an example that demonstrates this distinction:

let list = [4, 5, 6];

for (let i in list) {

  console.log(i); // "0", "1", "2",

}

for (let i of list) {

  console.log(i); // "4", "5", "6"

}

Another distinction is that for..in operates on any object; it serves as a way to inspect properties on this object. for..of on the other hand, is mainly interested in values of iterable objects. Built-in objects like Map and Set implement Symbol.iterator property allowing access to stored values.

let pets = new Set(["Cat", "Dog", "Hamster"]);

pets["species"] = "mammals";

for (let pet in pets) {

  console.log(pet); // "species"

}

for (let pet of pets) {

  console.log(pet); // "Cat", "Dog", "Hamster"

}