Iterators and generators are powerful features in TypeScript (and JavaScript) that allow for the creation and manipulation of sequences of values in a lazy, efficient manner, in this article we shall give a detailed account of what these concepts are all about and how to implement them in TypeScript.

Iterators

An iterator is an object that, when asked for a value, gives a sequence of values one by one, it adheres to the following protocol:

The object must have a next() method which will return an object also has two properties:

• value: The next value in a sequence.
• done: A Boolean indicating if this is true, whether the process is depleted or not.

Example: In this example, a SimpleIterator class has been used here to generate a series of numbers from start to end making use of next() method, the sequence is advanced each time the next() is called until it is finished.

class SimpleIterator {
    private current: number;
    private end: number;

    constructor(start: number, end: number) {
        this.current = start;
        this.end = end;
    }

    public next(): IteratorResult<number> {
        if (this.current <= this.end) {
            return { value: this.current++, done: false };
        } else {
            return { value: null, done: true };
        }
    }
}

const iterator = new SimpleIterator(1, 5);

let result = iterator.next();
while (!result.done) {
    console.log(result.value);
    result = iterator.next();
}  

Output:

1
2
3
4
5

Generators

Generators are a special kind of function that can stop, and then continue from where it stopped, using this kind of function you can make a function that implements an iterative algorithm only by writing just one function which doesn’t have to run its course at a go. They are defined by a function with an * in it and make use of the keyword yield in order to generate a set of numbers.

Example 1: The generatorFunction makes use of the function* syntax in order to return a sequence of numbers from 1 up to 5, here yield keyword will return each number and execution will be paused until next() is called again.

function* generatorFunction() {
    yield 1;
    yield 2;
    yield 3;
    yield 4;
    yield 5;
}

const generator = generatorFunction();

let result = generator.next();
while (!result.done) {
    console.log(result.value);
    result = generator.next();
}

Output:

1
2
3
4
5

Example 2: Generators can also accept parameters and return values. Fibonacci generator gives Fibonacci numbers up to a specified limit. Whenever next() is invoked it updates and outputs the next number in the Fibonacci sequence until it reaches the limit and stops there.

function* fibonacci(limit: number) {
    let a = 0, b = 1, count = 0;
    while (count < limit) {
        yield a;
        [a, b] = [b, a + b];
        count++;
    }
}

const fib = fibonacci(5);

for (const value of fib) {
    console.log(value);
}

Output:

0
1
1
2

Difference between Iterator and Generator