JavaScript generators are also consumers!

Yesterday, I introduced JavaScript generators motivated by the example of generating the Fibonacci sequence. This is the way generators are usually introduced: as generators of data. But, despite their name, generators can also be consumers of data!

Say you want to create a metric object that your application can pass numbers to with metric.log(4.3). The metric object will batch these into groups of 5 numbers, then send each batch to a central metric tracking system. Here’s how you could do it in traditional JavaScript:

function Metric() {
  this.batch = [];
  this.log = function(n) {
    this.batch.push(n);
    if (this.batch.length == 5) {
      send(this.batch);
      this.numbers = [];
    }
  }
}
const metric = new Metric();

metric.log(2.3);
metric.log(42);
// ...

But with a generator, you can implement the metric in one line:

function* Metric() {
  for (;;) send([yield, yield, yield, yield, yield]);
}
const metric = Metric();
metric.next();

metric.next(2.3);
metric.next(42);
// ...

How does this work? A generator is a kind of JavaScript iterator, meaning you can call .next() on it. Most standard iterators (like arrays) will ignore any arguments passed to .next(), but generators can accept arguments! This is how we get numbers into our metric object: metric.next(2.3).

Conversely, examples usually show the generator providing data, with expressions like yield x. But the yield expression can also consume data: the expression yield x evaluates to the value passed into the next .next() call.

Because we’re using yield as part of a complex expression above, evaluation order is important. In the expression [yield, yield, yield, yield, yield], which order will they yield in? The JS spec demands that expressions in an array literal are evaluated left-to-right. So the values in each batch will be in the order that they were produced by the application.

Notice, above, that we have to call metric.next() to initialize the metric. This pushes the generator’s program counter from the start of the function body to the first yield expression, making it ready to accept the first value in the array.

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Tagged #programming, #javascript. All content copyright James Fisher 2019. This post is not associated with my employer. Found an error? Edit this page.