---
title: "Tagged template literals"
url: "https://www.bekk.christmas/post/2021/22/tagged-template-literals"
authors: ["Mats Byrkjeland"]
published: "2021-12-22"
updated: "2024-11-07T13:05:29Z"
language: "en-US"
type: "article"
tags: ["JavaScript"]
---

# Tagged template literals

Template literals (`` `My name is ${name}` ``) have existed in JavaScript for a while. There is a good chance that it is your preferred way to concatenate strings in JavaScript. A slightly lesser known sub-feature of this is the _tagged_ template literal (``myTag`My name is ${name}`)``. What is it and why should you care?

## In short

Tagged template literals add a function-like layer to JavaScript template strings, giving access to the raw string parts and each substitution value separately. That makes it possible to transform or inspect interpolated values, and to build compact APIs that return more than just strings, as seen in tools like Styled Components, gql, and timeproxy.

- **For:** JavaScript developers
- **Good to know beforehand:** Basic knowledge of JavaScript template literals

_AI-generated_

In fact, the tag (`myTag` in the example above)  is just a function with a special signature. The first parameter is an array of strings. The following arguments are the substitution values provided. A tag like this, called `normal` here, would just concatenate the string as expected by a normal template string:

```jsx
function normal(strings, ...args) {
  return args.map((arg, i) => strings[i] + arg) + strings[args.length];
}

const a = normal`I am ${100} years old.`
// I am 100 years old.
```

We can have slightly more fun by adding some transformations like making this screaming function:

```jsx
function scream(strings, ...args) {
  const concatenated = args.map((arg, i) => strings[i] + arg) + strings[args.length];
  return concatenated.toUpperCase()
}

const a = scream`I am ${100} years old.`
// I AM 100 YEARS OLD.
```

Fun? Yes. But perhaps not very useful. And you could easily just call `.toUpperCase()` on the template string or create a normal function for it.

The great advantage of tagged template strings is that you can access the individual arguments passed to the string. Let's create a debug tag that prints a table of the arguments and their types if we're not in production mode:

```jsx
function debug(strings, ...args) {
  // If we're in production, do nothing
  if (process.env.NODE_ENV === "production") {
    return
  }
  
  // The normally concatenated string
  const concatenatedString =
    args.map((arg, i) => strings[i] + arg) + strings[args.length];

  // In non-production environments, log the string to console
  console.log(concatenatedString);

  // Then print a table of all the arguments!
  console.table(args.map((arg) => ({
    Value: arg, 
    Type: typeof arg,
  })));
}

debug`What is ${2} + ${"two"}? Beats me!`;

// What is 2 + two? Beats me!
// ┌─────────┬───────┬──────────┐
// │ (index) │ Value │   Type   │
// ├─────────┼───────┼──────────┤
// │    0    │   2   │ 'number' │
// │    1    │ 'two' │ 'string' │
//└─────────┴───────┴──────────┘
```

This is perhaps something we could use, leveraging the advantages of the tagged template string syntax. Notice that we don't need to return a string from this tag function. You can return whatever you want.

## Real-life usages

Tagged template literals are used in some popular libraries out there. You might recognize it from the popular CSS-in-JS library [Styled Components](https://styled-components.com/), where the tags are used to generate React components with certain styles:

```jsx
// Create a Wrapper component that'll render a <section> tag with some styles
// From https://styled-components.com/docs/basics
const Wrapper = styled.section`
  padding: 4em;
  background: papayawhip;
`;
```

If you have used GraphQL and the Apollo libraries, you might have encountered the [gql tag](https://github.com/apollographql/graphql-tag). It is a helpful utility for parsing your GraphQL queries, making them easier to work with compared to using simple strings.

Another library which supports the tagged template literal syntax is [timeproxy](https://github.com/selbekk/timeproxy). It lets you create human-readable time values with strings. And with tagged template literals, the syntax is quite compact and readable.

```jsx
import tp from 'timeproxy';

const TIMEOUT_LIMIT = tp`five seconds`;
const AGE_LIMIT = tp`1 week and 6 days`;
const SESSION_TIMEOUT = tp`${60} minutes`;
```

## Conclusion

Tagged template literals is a syntax that provides a new way of working with strings. Maybe you have some use case in your project that it is perfect for? At least now you know some of the magic behind this funky syntax when you see it!

## Relevant resources recommended by the author

- [Template literals on MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals): Documentation on MDN Web Docs

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