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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