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What are Tagged template literals?

Siddharth
13. Coding for a hobby
・2 min read

Quick refresher

  • Template literals are basically strings which are quoted with `` instead of single or double quotes.
  • You can interpolate data in them using ${} syntax. For example, if the variable data is "world", Hello ${data} will output "Hello World"

Tagged template literals are the next step from template literals. They are essentially functions which take a template literal as an argument, but in a special way.

How does this function work?

Example:

someFunction`some ${data} contained in a ${template} literal`
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As you can see, there are no parenthesis (brackets) for the function call. And the someFunction is basically a regular function.

Now let's see what kind of arguments the function gets:

// normal function to inspect the arguments const inspector = (...args) => { console.log(args) } // Let's try it! // First let's use an empty string console.log(inspector`hello world`)

Hmm. We get the string as an array. Now, let's just add some interpolation before I explain how this happened:

// normal function to inspect the arguments const inspector = (...args) => { console.log(args) } const life = 42; console.log(inspector`hello world, life is ${life}, and some more ${life} lives`)

Now we get:

  • An array of strings
  • 42
  • 42

Now let me explain.

Whenever a function is called using this syntax, it get's the following arguments:

  • an array of strings. The strings are obtained by splitting the string wherever there is an interpolation (`Hello ${people} and the world` => ["Hello ", " and the world"])
  • all the other arguments are the values of the interpolated data

Now, as this function is a normal function, it can return anything a normal function can.

Use cases

Now this may all seem pretty useless, but there are some uses. One could be to sanitize HTML:

function sanitize(strings, ...args) {
    // sanitize args
    // join the strings together
    // now return safe strings
}

let userMessages = ['hi', 'what are you up to?', '<script>alert("something evil")</script>']
const sanitized = `
  <div class="chat-list">
    <ul>
      ${userMessages.map(message => sanitize`<li>${message}</li>`)}
    </ul>
  </div>
`
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Another is CSS in JSX. I don't really use JSX, but I'm pretty sure that's what they do; Why else would they use an object?

Another (main) reason is – they are syntactic sugar.

Example: did you know you could write str.split("chars") as string.split`chars`? You just saved a few chars – that could be useful for some code golf?

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