Explain about Generators in JavaScript

Generators are used to define an iterative algorithm by writing a single function whose execution is not continuous.

How to define a generator?

'function*' declaration used to define a generator function.

Example

function* nextEvenNumber() {

  yield 2;

  yield 4;

  yield 6;

  yield 8;

}

HelloWorld.js

function* nextEvenNumber() {
  yield 2;
  yield 4;
  yield 6;
  yield 8;
}

var evenNumbers = nextEvenNumber();

console.log(evenNumbers.next());  // Get first yied, 2
console.log(evenNumbers.next());  // Get second yied, 4
console.log(evenNumbers.next());  // Get third yied, 6
console.log(evenNumbers.next());  // Get fourth yied, 8
console.log(evenNumbers.next());  // Get fifth yied, which is not there, undefined
console.log(evenNumbers.next());  // Get sixth yied, which is not there, undefined

Output

Object { value: 2, done: false }

Object { value: 4, done: false }

Object { value: 6, done: false }

Object { value: 8, done: false }

Object { value: undefined, done: true }

Object { value: undefined, done: true }

evenNumbers.next()

Returns a value yielded by the yield expression.

As you see the output, 'evenNumbers.next()' method returns an object that contains a value and a flag 'done'. Flag 'done' set to false, if next yield expression is available, else false.

Let’s enhance the above function to return infinite even numbers.

HelloWorld.js

function* nextEvenNumber() {
  var i = 0;
  
  while(true){
    yield i;
    i += 2;
  }
}

var evenNumbers = nextEvenNumber();

/* Print first 10 even numbers */
for(var i=0; i < 10; i++){
  console.log(evenNumbers.next().value);
}

Output

0

2

4

6

8

10

12

14

16

18

Can a generator function take arguments?

Yes, a generator function takes arguments.

function* name(param1, param2....paramN) {

   statements

}

HelloWorld.js

function* nextEvenNumber(maxNumber) {
  var i = 0;
  
  while(i <= maxNumber){
    yield i;
    i += 2;
  }
}

var evenNumbers = nextEvenNumber(10);

var obj = evenNumbers.next();

while(!obj.done){
  console.log(obj.value);
  obj = evenNumbers.next();
}

Output

0

2

4

6

8

10

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