[ES6] 15. Generators -- 2

Using for..of statement:

function* greeting(){
    console.log(`Generators are "lazy"`);
    yield "How";
    console.log(`I'm not called until the second next`);
    yield "are";
    console.log(`Call me before "you"?`);
    yield "you";
    console.log(`Called when "done"`);
}

let greeter = greeting();
for(word of greeter){
    console.log(word);
}

//How
//are
//you

You can see, it ouptu "How are you", because it grabs the value off of the next.

The same as:

let greeter = greeting();
console.log(greeter.next().value);
console.log(greeter.next().value);
console.log(greeter.next().value);
console.log(greeter.next().value);

 

Assign the yield to the variable:

function* greet(){
    let friendly = yield "How";
    console.log(friendly);
    yield "are";
    yield "you";
}

var greeter = greet();
console.log(greeter.next().value);
console.log(greeter.next().value);
console.log(greeter.next().value);

//How
//undefined
//are 
//you

When you assign yield to friendly, it doesn't console out "How" but undefined.

 

Why undefined?:

The way that this works is that the next step through the iteration, so if I say "The heck," will basically send this back through and assign it to this friendly. If I log this out now you'll see I get "How," and then "The heck." That means you can start building things through the iteration process.

 

Add param to the next():

function* greet(){
    let friendly = yield "How";
    console.log(friendly);
    yield "are";
    yield "you";
}

var greeter = greet();
console.log(greeter.next().value);
console.log(greeter.next("the hack").value);
console.log(greeter.next().value);

//How
//the hack
//are 
//you

 

function* greet(){
    let friendly = yield "How";
    friendly = yield friendly + "are";
    yield friendly + "you?";
}

var greeter = greet();
console.log(greeter.next().value);
console.log(greeter.next(" the heck ").value);
console.log(greeter.next(" silly ol'").value);

//How
// the heck are
// silly ol you

 

Cannot pass the param to the first next():

function* greet(){
    let friendly = yield "How";
    friendly = yield friendly + "are";
    yield friendly + "you?";
}

var greeter = greet();
console.log(greeter.next("first").value);

It will show error message:

　　TypeError: Sent value to newborn generator.

Because you haven't given this a chance to run and iterate and go to the next step where you could actually pass in a value.

 

总之：yield返回的值是下一个next中所传进来的值。

 

Generators also help you work with infinite sequences:

function* graph(){
    let x = 0;
    let y = 0;
    while(true){
        yield {x:x, y:y}
        x += 2;
        y += 1;
    }
}


var graphGenerator = graph();
console.log(graphGenerator.next().value);
console.log(graphGenerator.next().value);
console.log(graphGenerator.next().value);
console.log(graphGenerator.next().value);
console.log(graphGenerator.next().value);
console.log(graphGenerator.next().value);
console.log(graphGenerator.next().value);
console.log(graphGenerator.next().value);

I can safely yield this X and Y point knowing confidently that this stuff isn't going to evaluate until the next step through after the yield process.