JavaScript Generators Generator Functions
Example
A generator function is created with a function* declaration. When it is called, its body is not immediately executed. Instead, it returns a generator object, which can be used to "step through" the function's execution.
A yield expression inside the function body defines a point at which execution can suspend and resume.
function* nums() {
console.log('starting'); // A
yield 1; // B
console.log('yielded 1'); // C
yield 2; // D
console.log('yielded 2'); // E
yield 3; // F
console.log('yielded 3'); // G
}
var generator = nums(); // Returns the iterator. No code in nums is executed
generator.next(); // Executes lines A,B returning { value: 1, done: false }
// console: "starting"
generator.next(); // Executes lines C,D returning { value: 2, done: false }
// console: "yielded 1"
generator.next(); // Executes lines E,F returning { value: 3, done: false }
// console: "yielded 2"
generator.next(); // Executes line G returning { value: undefined, done: true }
// console: "yielded 3"
Early iteration exit
generator = nums();
generator.next(); // Executes lines A,B returning { value: 1, done: false }
generator.next(); // Executes lines C,D returning { value: 2, done: false }
generator.return(3); // no code is executed returns { value: 3, done: true }
// any further calls will return done = true
generator.next(); // no code executed returns { value: undefined, done: true }
Throwing an error to generator function
function* nums() {
try {
yield 1; // A
yield 2; // B
yield 3; // C
} catch (e) {
console.log(e.message); // D
}
}
var generator = nums();
generator.next(); // Executes line A returning { value: 1, done: false }
generator.next(); // Executes line B returning { value: 2, done: false }
generator.throw(new Error("Error!!")); // Executes line D returning { value: undefined, done: true}
// console: "Error!!"
generator.next(); // no code executed. returns { value: undefined, done: true }
