JavaScript generators are amazing. A brilliant concept infinitely useful on paper.

They let you write infinite loops that terminate. Yield values from functions before they finish. Build lazy data structures. Lazy evaluation programming!

A functional programmer’s wet dream. 😍

And I bet you’ve never used a JavaScript generator even once did you? Do you even know the syntax?

I bet you don’t.

Turns out generators aren’t very useful in practice. 😢

But yesterday, after years of knowing about generators, years of itching to use them, years of … I FOUND A USE! A real world practical use-case where generators actually make your code better. 😱

We were adding automatic linking of twitter usernames to my TechLetterApp project. Find a piece of text, parse out @username instances, turn into links.

Easy right?

I thought so too. Complications around inserting complex nodes into an abstract syntax tree (AST) aside, parsing those usernames is hard to do elegantly.

Unless you use generators 😛

Here’s how we did it 👇

First, you create a regex that matches @ followed by a series of word symbols.

const UserRegex = new RegExp(/@(\w+)/, "g");

Then you create a generator that runs this regex in a loop. Each UserRegex.exec() returns the next match.

function* getUsernames(string) {
    let match = null;

    do {
        match = UserRegex.exec(string);
        if (match) {
            yield match;
        }
    } while (match);
}

We have a *getUsernames generator that takes a string. The asterisk notation changes a function to a generator.

Inside, a loop runs as long as match has a value. Assigned on each iteration as a UserRegex.exec call.

We yield existing values on every loop iteration. Last one will be null and we don’t want to return those.

Yield is how you return values from generators. Notice that even after returning a value, the generator keeps running. That’s the generator magic 🧙‍♀️

You can now find all usernames in a string with a loop.

const string = "this is a test with @swizec and @kyleshevlin, maybe @lukeed05"

for (const username of getUsernames(string)) {
  console.log(username)
}

That outputs

["@swizec", "swizec"]
["@kyleshevlin", "kyleshevlin"]
["@lukeed05", "lukeed05"]

Here’s a CodeSandbox you can play with

Cheers,
~Swizec

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