yield

The yield keyword is used to pause and resume a generator function.

[rv] = yield [expression]

expression Optional

Defines the value to return from the generator function via the iterator protocol. If omitted, undefined is returned instead.

rv Optional

Retrieves the optional value passed to the generator's next() method to resume its execution.

The yield keyword pauses generator function execution 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.

yield can only be called directly from the generator function that contains it. It cannot be called from nested functions or from callbacks.

The yield keyword causes the call to the generator's next() method to return an IteratorResult object with two properties: value and done. The value property is the result of evaluating the yield expression, and done is false, indicating that the generator function has not 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:

• A yield, which causes the generator to once again pause and return the generator's new value. The next time next() is called, execution resumes with the statement immediately after the yield.
• throw is used to throw an exception from the generator. This halts execution of the generator entirely, and execution resumes in the caller (as is normally the case when an exception is thrown).
• The end of the generator function is reached. In this case, execution of the generator ends and an IteratorResult is returned to the caller in which the value is undefined and done is true.
• A return statement is reached. In this case, execution of the generator ends and an IteratorResult is returned to the caller in which the value is the value specified by the return statement and done is true.

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

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

The following code is the declaration of an example generator function.

function* countAppleSales () {
  let saleList = [3, 7, 5]
  for (let i = 0; i < saleList.length; i++) {
    yield saleList[i]
  }
}

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

let appleStore = countAppleSales()  // Generator { }
console.log(appleStore.next())      // { value: 3, done: false }
console.log(appleStore.next())      // { value: 7, done: false }
console.log(appleStore.next())      // { value: 5, done: false }
console.log(appleStore.next())      // { value: undefined, done: true }

You can also send a value with next(value) into the generator. 'step' evaluates as a return value in this syntax [rv] = yield [expression]

function* counter(value) {
 let step;

 while (true) {
   step = yield ++value;

   if (step) {
     value += step;
   }
 }
}

const generatorFunc = counter(0);
console.log(generatorFunc.next().value);   // 1
console.log(generatorFunc.next().value);   // 2
console.log(generatorFunc.next().value);   // 3
console.log(generatorFunc.next(10).value); // 14
console.log(generatorFunc.next().value);   // 15
console.log(generatorFunc.next(10).value); // 26

• The Iterator protocol
• function*
• function* expression
• yield*