yield

Summary

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

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 object with two parameters, value and done. value is the result of evaluating the yield expression, and done is a Bool 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 for 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 2, 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

See also

• Iterators
• function*
• yield*