Generators

Generators

Also called function *, generators allow you to create functions whose execution can be paused and then later resumed maintaining the state between pause-resume transitions. The value returned from a generator is called an iterator and can be used to control this pause-resume transition.

Here is a simple example of a generator function that generates an infinite list of whole numbers.

function* wholeNumbers() {
    var current = 0;
    while(true) {
        yield current++;
    }
}

The yield contextual keyword is used to return control from a generator (effectively pausing function execution) along with an optional value (here current). You can get access to this value using the iterator's .next() member function, this is shown below:

function* wholeNumbers() {
    var current = 0;
    while(true) {
        yield current++;
    }
}
var iterator = wholeNumbers();
console.log(iterator.next()); // 0
console.log(iterator.next()); // 1
console.log(iterator.next()); // 2
// so on till infinity....

Now that you have seen function*, yield and .next() we can dig deeper.

Catching Errors

Any errors thrown (intentially using throw or unintentionally due to error) from the generator can be caught using try/catch just like normal function executions. This is demonstrated below:

function* wholeNumbers() {
    var current = 0;
    while(true) {
      if (current === 3)
        throw new Error('3 is the magic number');
      else
        yield current++;
    }
}
var iterator = wholeNumbers();
console.log(iterator.next()); // 0
console.log(iterator.next()); // 1
console.log(iterator.next()); // 2
try {
    console.log(iterator.next()); // Will throw an error
}
catch(ex) {
    console.log(ex.message); // 3 is the magic number
}

Controlling function execution externally

The iterator returned from the generator function can be used to control the state inside the generator function as well.

// TODO: example

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