LayaBox---TypeScript---Iterator and generator

1. Iterability

When an object implements the Symbol.iterator property, we consider it to be iterable.Some built-in types such as Array, Map, Set, String, Int32Array, Uint32Array, etc. have all implemented their own Symbol.iterator.The Symbol.iterator function on the object is responsible for returning the value for iteration.

1.1 for..of statement

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

1.2 for..of vs. for..in statement

for..of and for..in both iterate over a list; but the values ​​used for iteration are different,

Difference 1:for..in iterates over the object's key list, while for..of iterates over the object's key corresponding value.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"}Difference two:for..in can operate on any object and provides a way to view the properties of the object.for..of is concerned with iterating over the values ​​of the object.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"}

2. Code generation

When the build target is ES5 or ES3, Iterators are only allowed on Array types.Using the for..of statement on non-array values ​​will get an error, even if those non-array values ​​already implement the Symbol.iterator property.

The compiler will generate a simple for loop as a for..ofloop

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);}

When the target is an engine compatible with ECMAScipt 2015, the compiler will generate the for..of built-in iterator implementation of the corresponding engine.