Introduction

Generators is a new feature from ES6 which can help us to maintain the internal state (pause and resume)

The Iterator Protocol

• The iterator protocol means a process for defining how an object will iterate.This is done through implementing the .next() method.

const list = [0, 1, 2, 3, 4, 5];
const arrayIterator = list[Symbol.iterator]();

console.log(arrayIterator.next());
console.log(arrayIterator.next());
console.log(arrayIterator.next());
/*
will print:
Object {value: 0, done: false}
Object {value: 1, done: false}
Object {value: 2, done: false}
*/

• value: represent the next value in the sequence
• done: represent if the iterator is done ging through the sequence

Pausable Functions

• When a generator is invoked, it doesn't actually run any of the code inside the function. Instead, it creates and returns an iterator.
• The function will be transformed to a pausable function if we put asterisk after the function keyword.
• yield is the keyword that causes the generator to pause
• The state machine(Generators) will transform the state: initial state -> resume -> pause (state 1) -> resume -> pause (state 2) -> resume -> .... -> end (state N)

Sending data into/out of a Generator

• yield is used to send data outside the generator
• .next() method is used to send data inside the generator

function* gen3() {
    for (let i = 0; i< 10; i++) {
        let value = yield i
        console.log(`sending data into generator: ${value}`)
    }
}


var g = gen3()
console.log(`sending data out of generator: ${g.next().value}`)
console.log(`sending data out of generator: ${g.next(10).value}`)
console.log(`sending data out of generator: ${g.next(20).value}`)
/*
will print:
sending data out of generator: 0
sending data into generator: 10
sending data out of generator: 1
sending data into generator: 20
sending data out of generator: 2
*/

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References

• JavaScript ES6 Generators 生成器
• 你懂 JavaScript 嗎？#25 產生器（Generator）
• Udacity ES6 class