TypeScript iterators and generators

Iterative

When an object implements the Symbol.iterator we consider it it iterative. S ome built-in Array Map Set String Int32Array Uint32Array have Symbol.iterator The Symbol.iterator for returning the value for iteration.

for..of statement

for..of traverses iterative objects and calls the Symbol.iterator object. H ere's how to use for..of simple example of of of:

let someArray = [1, "string", false];

for (let entry of someArray) {
    console.log(entry); // 1, "string", false
}

for..of vs. for..in statement

for..of and for..in can iterate on a list, but the value used for iteration is different, for..in iterates is a list of the keys of the for..of the value corresponding to the key of the iterative object.

The following example shows the difference between the two:

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 difference is for..in can manipulate any object; B ut for..of on the value of the iterative object. The Map Set and Symbol.iterator method so that we can access the values they save.

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"
}

Code generation

Targets are ES5 and ES3

When the build target is ES5 or ES3, the iterator is only allowed on the Array type. U se for: on non-array for..of statement gets an error, even if these non-array values have implemented the Symbol.iterator

The compiler generates a for loop as for..of such as:

let numbers = [1, 2, 3];
for (let num of numbers) {
    console.log(num);
}

The generated code is:

var numbers = [1, 2, 3];
for (var _i = 0; _i < numbers.length; _i++) {
    var num = numbers[_i];
    console.log(num);
}

Target ECMAScript 2015 or higher

When the target is an ECMAScipt 2015-compatible engine, the compiler generates the for..of built-in iterator implementation.