Detailed explanation of Generator generator in Javascript

What is a Generator?

A generator is some code that runs inside a function

• After returning a value, it pauses itself, and -

• The calling program can ask to unpause and return another value

This "return" is not a traditional slave functionreturn. So it was given a special name -yield.

Generator syntax varies from language to language. Javascript's generator syntax is similar to PHP's, but it's different enough that if you expect them to do the same thing, you'll end up getting very confused.

In javascript, if you want to use a generator, you need to:

• Define a special generator function

• Call this function to create a generator object

• Use the generator object in a loop, or directly call itsnextmethod

We use the following simple program as a starting point and perform each of the following steps:

// File: sample-program.js function *createGenerator() { for(let i=0;i<20;i++) { yield i } } const generator = createGenerator() console.log(generator.next()) console.log(generator.next())

If you run this code, you will get the following output:

$ node sample-program.js { value: 0, done: false } { value: 1, done: false }

Here I go Explain how the program works.

Generator function

First, there is a definition of the generator function in the code:

function* createGenerator() { for(let i=0;i<20;i++) { yield i } }

functionfollowed by*Tells javascript that this is a generator function. The following writings are all valid definitions of generator functions.

function*createGenerator function* createGenerator function *createGenerator

*andare not part of thefunction name. Instead, thefunction*symbols define generators.

Call the generator function

After defining the generator function, we name it a function with another name.

// 注意：当调用时，没有 *。 * 不是函数名称的一部分 // `function *` 是用于定义生成器函数的符号 const generator = createGenerator()

But remember:createGeneratorThe functionhas noreturn value. This is because generator functions don't have traditional return values. In contrast, when you call a generator function directly, italwaysreturns an instantiatedGeneratorobject.

This generator object has anextmethod. Callingnextwill run the code inside the generator function.

function* createGenerator() { for(let i=0;i<20;i++) { yield i } }

This is important enough to call it again. Calling a generator functiondirectly will notrun any code within the generator function. Instead, create a generator object. It callsnexton the generator object, thereby calling the code in the generator function.

The first timenextis called on a generator object, the internal code will run until theyieldstatement occurs. Once execution reachesyield, javascript willpausethe execution of that code, andnextwill return (i.e. to you, oryield) An object containing the values in theyieldrows.

When you callnexta second time (or third, fourth or even more times), the code will unpause and continue running (as in the last call) where it was interrupted). The variable (such asiin this example) will retain its value. When the code reaches anotheryieldstatement, the function pauses again and returns an object containing the yield value.

This is why we have to call twicenext

console.log(generator.next()) console.log(generator.next())

will get the following output:

{ value: 0, done: false } { value: 1, done: false }

After the code in the generator function is executed, Any future calls tonextwill return an object with the valueundefinedanddoneset totrue.

{ value: undefined, done: true }

Generators and Loops

Althoughnextcan be called manually on the generator object, we mainly want to use it in a loop. Take a look at this slightly modified program.

// File: sample-program.js @highlightsyntax@jscript function *createGenerator() { for(let i=0;i<5;i++) { yield i } } const generator = createGenerator() for(const value of generator) { console.log(value) }

When using a generator object in afor...ofloop,nextis called on the generator object each time through the loop, with the resulting value Populate the variable (valueabove). Running this program will output the following:

$ node sample-program.js 0 1 2 3 4

In the next article, we will take a deeper look atfor ... ofloops and explore how to provide a built-in Method to loop through any object in javascript

English original address: https://alanstorm.com/javascript-generators/

Author: Alan Storm

Translation Address: https://segmentfault.com/a/1190000023924834

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