Using the function* Declaration in JavaScript

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().

<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().

<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