Reprint: "TypeScript Chinese Introductory Course" 15, can be iterated

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The scalability of the iteration

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

for..ofStatement

for..ofIterates over an object that is iterated, calling properties on the object Symbol.iterator . The following is a simple example used on an array for..of :

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

for..ofVs. for..inStatement

for..ofAnd for..in can iterate over a list, but the values used for the iteration are different, the for..in iteration is the list of keys for the object, and the for..of value corresponding to the numeric key of the Iteration 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"

One difference is that for..in you can manipulate any object; it provides a way to view the properties of an object. But for..in focus on the value of the Iteration object. Built-in Objects Map and Set properties have been implemented Symbol.iterator so that we can access the values they hold.

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, iterators are only allowed on Array types. Using a statement on a non-array value for..of will get an error, even if these non-array values have already implemented the Symbol.iterator property.

The compiler generates a simple for loop as a for..of loop, for example:

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 is ECMAScript 2015 or higher

When the target is an engine that is compatible with Ecmascipt 2015, the compiler generates a for..of built-in iterator implementation for the corresponding engine.

Reprint: "TypeScript Chinese Introductory Course" 15, can be iterated