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
