yield

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

Syntax

yield [[expression]];

expression

The expression to return. If omitted, undefined is returned instead.

Description

The yield keyword causes generator function execution to pause and return the current value of the expression following the yield keyword. 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, code execution for the generator cannot resume unless invoked externally by calling the generator's next() method. This allows direct control of the generator's execution and incremental return values.

Examples

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

Specifications

Browser compatibility

Firefox-specific notes

• Starting with Gecko 29 (Firefox 29 / Thunderbird 29 / SeaMonkey 2.26), the completed generator function no longer throws a TypeError "generator has already finished". Instead, it returns an IteratorResult object like { value: undefined, done: true } (bug 958951).
• Starting with Gecko 33 (Firefox 33 / Thunderbird 33 / SeaMonkey 2.30), the parsing of the yield expression has been updated to conform with the latest ES6 specification (bug 981599):
  • The expression after the yield keyword is optional and omitting it no longer throws a SyntaxError: function* foo() { yield; }

See also

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