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.