Using the function* Declaration in JavaScript

Last Updated : 30 Dec, 2022

The function* declaration is used to define a generator that returns a Generator object. Generators are very powerful for asynchronous programming as they aim to resolve callback problems.

In a generator, the yield keyword is used instead of return. The yield statement suspends the function’s execution and sends a value back to the caller, but retains enough state to enable the function to resume from the last execution state. The function, therefore, continues execution immediately after the last yield run. The next() method is used to return an object with two properties, done and value, and can be used to proceed to the next state of the generator.

Syntax:

function* function_name(param1, param2...)  {    function body  }

The below examples demonstrate the use of the function* declaration.

Example 1: This example shows the declaration of a function * and the use of yield and next().

JavaScript

<script>     // Declaring a generator function     function* generator(i) {       yield i;       yield i + 50;       yield i + 100;     }          // Initialize the generator     const generate = generator(50);          // Console out the object returned     // by the next() method     let nextValObj = generate.next();          // Console out the value of the object     console.log(nextValObj.value);          // Console out next iterations     // of the generator     console.log(generate.next().value);     console.log(generate.next().value);          // This value would be undefined as     // the last defined one is i + 100     console.log(generate.next().value); </script>

Output:

50  100  150  undefined

Example 2: This example shows the declaration of a function * and the use of yield and next().

JavaScript

<script>     function* powerup(n) {       for (let num = n; ; num *= n) {              // Yield out the current number         yield num;       }     }     for (let power of powerup(5)) {            // Break if the number is       // more than 1024       if (power > 1024)         break;            console.log("Yielded:", power)     } </script>

Output:

Yielded: 5  Yielded: 25  Yielded: 125  Yielded: 625

What is the difference between call and apply in JavaScript ?

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