在学习yield之前，您需要了解生成器。生成器是使用函数*语法创建的。Generator函数不执行代码，而是返回一种称为Generator的迭代器。当使用下一个方法给出一个值时，生成器函数将继续执行，直到遇到yield关键字。使用yield返回一个包含两个值的对象，一个是value，另一个是done(布尔值)。该值可以是数组、对象等。

不要忘记使用非常有用的“x of generator”语法来遍历生成器。根本不需要使用next()函数。

function* square(x){
    for(i=0;i<100;i++){
        x = x * 2;
        yield x;        
    }   
}

var gen = square(2);
for(x of gen){
   console.log(x);
}

使用yield关键字的斐波那契序列生成器。

function* fibonacci() {
    var a = -1, b = 1, c;
    while(1) {
        c = a + b;
        a = b;
        b = c;
        yield c;
    }   
}

var fibonacciGenerator = fibonacci();
fibonacciGenerator.next().value; // 0 
fibonacciGenerator.next().value; // 1
fibonacciGenerator.next().value; // 1
fibonacciGenerator.next().value; // 2 

它用于迭代器生成器。基本上，它允许您使用过程代码制作(可能是无限的)序列。请参阅Mozilla的文档。

举个简单的例子:

const strArr = ["red", "green", "blue", "black"];

const strGen = function*() {
    for(let str of strArr) {
        yield str;
    }
};

let gen = strGen();

for (let i = 0; i < 5; i++) {
    console.log(gen.next())
}

//prints: {value: "red", done: false} -> 5 times with different colors, if you try it again as below:

console.log(gen.next());

//prints: {value: undefined, done: true}

在我看来，MDN文档非常好。

The function containing the yield keyword is a generator. When you call it, its formal parameters are bound to actual arguments, but its body isn't actually evaluated. Instead, a generator-iterator is returned. Each call to the generator-iterator's next() method performs another pass through the iterative algorithm. Each step's value is the value specified by the yield keyword. Think of yield as the generator-iterator version of return, indicating the boundary between each iteration of the algorithm. Each time you call next(), the generator code resumes from the statement following the yield.