A lot of writing in the last two years Generator+co, Use it to write code similar to synchronization
But actually ,Generator It's not made for this , Otherwise, there will be no later asyncawait
Generator Is a function that can be suspended , And when to recover , It's up to the caller
I hope this article can help you understand Generator What is it , And how to use it

Show me a picture of Generator The understanding of the :

A coffee machine , Although I don't drink coffee , Unfortunately, we can't find a machine to make Wang Laoji -.-

What I understand Generator The coffee machine is like this :

  1. First , We put some coffee beans in the machine
  2. When we want coffee , You can press the switch (gen.next()), The machine starts to grind the coffee beans 、 The coffee 、 And then there's coffee
  3. After a full cup of coffee , The valve will close automatically (yield)
  4. If you put a lot of coffee beans in the machine at first , here , There will still be some left in the machine , Next time you want to drink, you can continue to press the switch , perform ( The bean grinding 、 The coffee 、 Pick up the coffee ) This set of operations

take Generator Let's realize the above coffee machine :

function * coffeeMachineGenerator (beans) {
do {
yield cookCoffee()
} while (--beans) // The coffee
function cookCoffee () {
console.log('cooking') return 'Here you are'
}
} // Put coffee beans in the coffee machine
let coffeeMachine = coffeeMachineGenerator(10) // I want to have coffee
coffeeMachine.next() // I am here 3 I'll have coffee in seconds
setTimeout(() => {
coffeeMachine.next()
}, 3 * 1e3)

After the code runs , We'll get one first cooking Of log,
And then in 3s I'll get another one after that log.

This explains Generator What is it? :
An iterator that can pause
call next To get data ( We decide for ourselves whether or not to make coffee
In case of yield After that, the execution of the function will stop ( It's a full cup , The valve is closed
Let's decide when to run the rest of the code next When do you want to drink and then cook

This is a Generator The most important feature of , We only get the next value when we really need it , Instead of getting all the values at once

Generator The grammar of

Statement Generator There are many ways to function , The most important thing is , stay function After the keyword, add a *

function * generator () {}
function* generator () {}
function *generator () {} let generator = function * () {}
let generator = function* () {}
let generator = function *() {} // Examples of mistakes
let generator = *() => {}
let generator = ()* => {}
let generator = (*) => {}

perhaps , Because it's a function , It can also exist as an attribute of an object :

class MyClass {
* generator() {}
*generator2() {}
} const obj = {
*generator() {}
* generator() {}
}

generator Initialization and reuse of

One Generator Function by calling the method twice , Two completely independent State machine
therefore , Save the current Generator Objects matter :

function * generator (name = 'unknown') {
yield `Your name: ${name}`
} const gen1 = generator()
const gen2 = generator('Niko Bellic') gen1.next() // { value: Your name: unknown , done: false}
gen2.next() // { value: Your name: Niko Bellic, done: false}

Method: next()

Most commonly used next() Method , Whenever it is called , Will get the return object of the next output ( The call after code execution will always return {value: undefined, done: true}).

next Always returns an object , Contains two property values :
valueyield The value of the expression after the keyword
done : If there is no more yield Keyword. , Will return true .

function * generator () {
yield 5
return 6
} const gen = generator() console.log(gen.next()) // {value: 5, done: false}
console.log(gen.next()) // {value: 6, done: true}
console.log(gen.next()) // {value: undefined, done: true}
console.log(gen.next()) // {value: undefined, done: true} -- Subsequent calls will also be the result

Use as an iterator

Generator The function is an iterative , therefore , We can go straight through for of To use it .

function * generator () {
yield 1
yield 2
return 3
} for (let item of generator()) {
item
} //
//

return Not participating in iteration
Iterations perform all of the yield, in other words , After the iteration Generator Object will no longer return any valid values

Method: return()

We can call... Directly on the iterator object return(), To terminate subsequent code execution .
stay return After all next() Calls will return {value: undefined, done: true}

function * generator () {
yield 1
yield 2
yield 3
} const gen = generator() gen.return() // {value: undefined, done: true}
gen.return('hi') // {value: "hi", done: true}
gen.next() // {value: undefined, done: true}

Method: throw()

Calling throw() After that, it will also terminate all yield perform , An exception is thrown at the same time , Need to pass through try-catch To receive :

function * generator () {
yield 1
yield 2
yield 3
} const gen = generator() gen.throw('error text') // Error: error text
gen.next() // {value: undefined, done: true}

Yield The grammar of

yield Its grammar is a bit like return, however ,return It returns the result at the end of the function call
And calling return Nothing else will be done after that

function method (a) {
let b = 5
return a + b
// The following two sentences will never be executed
b = 6
return a * b
} method(6) //
method(6) //

and yield It's not the same

function * yieldMethod(a) {
let b = 5
yield a + b
// On the second execution `next` when , The next two lines will execute
b = 6
return a * b
} const gen = yieldMethod(6)
gen.next().value //
gen.next().value //

yield*

yield* Used to put a Generator Put it on the other Generator Execute in function .
It's kind of like [...] The function of :

function * gen1 () {
yield 2
yield 3
} function * gen2 () {
yield 1
yield * gen1()
yield 4
} let gen = gen2() gen.next().value //
gen.next().value //
gen.next().value //
gen.next().value //

yield The return value of

yield Can receive the return value , The return value can be used in subsequent code
A strange way of writing

function * generator (num) {
return yield yield num
} let gen = generator(1) console.log(gen.next()) // {value: 1, done: false}
console.log(gen.next(2)) // {value: 2, done: false}
console.log(gen.next(3)) // {value: 3, done: true }

We're calling for the first time next When , The code executes to yield num, Return at this time num
Then we call next(2), The code executes yield (yield num), And the value that we return is what we're doing next The parameters passed in the , As yield num The return value of exists .
And finally next(3), This part of the code is executed return (yield (yield num)), The second time yield Return value of expression .

Some actual use scenarios

All of the examples above are based on a known number of Generator Functional , But if you need an unknown number of Generator, Just create an infinite loop .

A simple random number generation

For example, we will achieve the acquisition of a random number :

function * randomGenerator (...randoms) {
let len = randoms.length
while (true) {
yield randoms[Math.floor(Math.random() * len)]
}
} const randomeGen = randomGenerator(1, 2, 3, 4) randomeGen.next().value // Returns a random number

Instead of some recursive operations

The most famous Fibonacci number , Basically, you choose to use recursive implementation
But combined with Generator in the future , You can use an infinite loop to achieve :

function * fibonacci(seed1, seed2) {
while (true) {
yield (() => {
seed2 = seed2 + seed1;
seed1 = seed2 - seed1;
return seed2;
})();
}
} const fib = fibonacci(0, 1);
fib.next(); // {value: 1, done: false}
fib.next(); // {value: 2, done: false}
fib.next(); // {value: 3, done: false}
fib.next(); // {value: 5, done: false}
fib.next(); // {value: 8, done: false}

And async/await The combination of

Reiterated , I personally don't think async/await yes Generator The grammar sugar of ..

If it's children's shoes written on the front end , Basically, you will encounter the time when you load data by paging
If combined with Generator+asyncawait, We can do this :

async function * loadDataGenerator (url) {
let page = 1 while (true) {
page = (yield await ajax(url, {
data: page
})) || ++page
}
} // Use setTimeout Impersonate asynchronous request
function ajax (url, { data: page }) {
return new Promise((resolve) => {
setTimeout(_ => {
console.log(`get page: ${page}`);
resolve()
}, 1000)
})
} let loadData = loadDataGenerator('get-data-url') await loadData.next()
await loadData.next() // force load page 1
await loadData.next(1)
await loadData.next() // get page: 1
// get page: 2
// get page: 1
// get page: 2

In this way, we can implement a paging control function in a few lines of code .
If you want to load a specific page number from , Direct will page Pass in next that will do .

Notes

Generator There are more ways to use it ,( Realize asynchronous process control 、 Read data on demand )
Personally think that ,Generator The advantage of is the lazy execution of the code ,Generator What is achieved , We can do it without it , Just use Generator after , Can make code more readable 、 The process becomes clearer 、 More focused on the implementation of logic .

If there's something you don't understand or Some mistakes in the article , Welcome to point out

Reference material

  1. Javascript (ES6) Generators — Part I: Understanding Generators
  2. What are JavaScript Generators and how to use them

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