Composing Generator functions in JavaScript

Published August 12, 2020

Composing generators in JavaScript is a way of combining and using a generator function inside another generator function body.

Let's understand this with the help of an example,

Suppose we have a generator function which generates number from a starting number to an ending number.

// Generates numbers from start to end
function* GenerateNumbers(start, end) {
  for (let i = start; i <= end; i++) {
    yield i;
  }
}

Now let's create another generator function that uses the above GenerateNumbers() generator function.

The new generator function first generates values from 1 to 100 and then from 300 to 400.

Let's use the GenerateNumbers() inside the new generator function to generate those numbers.

// Generates numbers from start to end
function* GenerateNumbers(start, end) {
  for (let i = start; i <= end; i++) {
    yield i;
  }
}

// Sequence Generator 1 - 100 and 300 to 400
function* SequenceGenerator() {
  // embed another generator function here
}

Inside this function, we need to use the yield* special keyword to compose or embed the generator function within a generator function.

// Generates numbers from start to end
function* GenerateNumbers(start, end) {
  for (let i = start; i <= end; i++) {
    yield i;
  }
}

// Sequence Generator 1 - 100 and 300 to 400
function* SequenceGenerator() {
  yield* GenerateNumbers(1, 100);
  yield* GenerateNumbers(300, 400);
}

Let's use the for..of looping statement to print the values to the console.

// Generates numbers from start to end
function* GenerateNumbers(start, end) {
  for (let i = start; i <= end; i++) {
    yield i;
  }
}

// Sequence Generator 1 - 100 and 300 to 400
function* SequenceGenerator() {
  yield* GenerateNumbers(1, 100);
  yield* GenerateNumbers(300, 400);
}

for (let number of SequenceGenerator()) {
  console.log(number);
}

/* 
OUTPUT
------

1
2
3
.
.
.
100
300
301
302
.
.
.
400

*/

As you can see that first the numbers from 1 to 100 are printed then numbers from 300 to 400 are printed.

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