The yield keyword is used to pause and resume a generator function (function* or legacy generator function).


[rv] = yield [expression];
Defines the value to return from the generator function via the iterator protocol. If omitted, undefined is returned instead.
Lets the generator capture the value of expression for use the next time it resumes execution.


The yield keyword causes generator function execution to pause and the value of the expression following the yield keyword is returned to the generator's caller. It can be thought of as a generator-based version of the return keyword.

The yield keyword actually returns an IteratorResult object with two properties, value and done. The value property is the result of evaluating the yield expression, and done is a Boolean indicating whether or not the generator function has fully completed.

Once paused on a yield expression, the generator's code execution remains paused until the generator's next() method is called. Each time the generator's next() method is called, the generator resumes execution and runs until it reaches one of the following:

If an optional value is passed to the generator's next() method, that value becomes the value returned by the generator's next yield operation.

Between the generator's code path, its yield operators, and the ability to specify a new starting value by passing it to, generators offer enormous power and control.


The following code is the declaration of an example generator function, along with a helper function.

function* foo(){
  var index = 0;
  while (index <= 2) // when index reaches 3, 
                     // yield's done will be true 
                     // and its value will be undefined;
    yield index++;

Once a generator function is defined, it can be used by constructing an iterator as shown.

var iterator = foo();
console.log(; // { value: 0, done: false }
console.log(; // { value: 1, done: false }
console.log(; // { value: 2, done: false }
console.log(; // { value: undefined, done: true }

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