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.