# A guide to JavaScript Template Literals

> Introduced in ES2015, aka ES6, Template Literals offer a new way to declare strings, but also some new interesting constructs which are already widely popular.

Author: Flavio Copes | Published: 2018-03-15 | Canonical: https://flaviocopes.com/javascript-template-literals/

<!-- TOC -->

- [Introduction to Template Literals](#introduction-to-template-literals)
- [Multiline strings](#multiline-strings)
- [Interpolation](#interpolation)
- [Template tags](#template-tags)

<!-- /TOC -->

## Introduction to Template Literals

Template Literals are a new ES2015 / ES6 feature that allows you to work with strings in a novel way compared to ES5 and below.

The syntax at a first glance is very simple, just use backticks instead of single or double quotes:

```js
const a_string = `something`
```

They are unique because they provide a lot of features that normal strings built with quotes do not, in particular:

- they offer a great syntax to define multiline strings
- they provide an easy way to interpolate variables and expressions in strings
- they allow you to create DSLs with template tags (DSL means domain specific language, and it's for example used in [React](https://flaviocopes.com/react/) by Styled Components, to define CSS for a component)

Let's dive into each of these in detail.

## Multiline strings

Pre-ES6, to create a string spanning over two lines you had to use the `\` character at the end of a line:

```js
const string =
  'first part \
second part'
```

This allows to create a string on 2 lines, but it's rendered on just one line:

`first part second part`

To render the string on multiple lines as well, you explicitly need to add `\n` at the end of each line, like this:

```js
const string =
  'first line\n \
second line'
```

or

```js
const string = 'first line\n' + 'second line'
```

Template literals make multiline strings much simpler.

Once a template literal is opened with the backtick, you just press enter to create a new line, with no special characters, and it's rendered as-is:

```js
const string = `Hey
this

string
is awesome!`
```

Keep in mind that space is meaningful, so doing this:

```js
const string = `First
                Second`
```

is going to create a string like this:

```
First
                Second
```

an easy way to fix this problem is by having an empty first line, and appending the trim() method right after the closing backtick, which will eliminate any space before the first character:

```js
const string = `
First
Second`.trim()
```

## Interpolation

Template literals provide an easy way to interpolate variables and expressions into strings.

You do so by using the `${...}` syntax:

```js
const myVariable = 'test'
const string = `something ${myVariable}` //something test
```

inside the `${}` you can add anything, even expressions:

```js
const string = `something ${1 + 2 + 3}`
const string2 = `something ${doSomething() ? 'x' : 'y'}`
```

## Template tags

Tagged templates is one feature that might sound less useful at first for you, but it's actually used by lots of popular libraries around, like [Styled Components](https://flaviocopes.com/styled-components/) or [Apollo](https://flaviocopes.com/apollo/), the [GraphQL](https://flaviocopes.com/graphql/) client/server lib, so it's essential to understand how it works.

In Styled Components template tags are used to define CSS strings:

```js
const Button = styled.button`
  font-size: 1.5em;
  background-color: black;
  color: white;
`
```

In Apollo template tags are used to define a [GraphQL](https://flaviocopes.com/graphql/) query schema:

```js
const query = gql`
  query {
    ...
  }
`
```

The `styled.button` and `gql` template tags highlighted in those examples are just **functions**:

```js
function gql(literals, ...expressions) {}
```

this function returns a string, which can be the result of _any_ kind of computation.

`literals` is an array containing the template literal content tokenized by the expressions interpolations.

`expressions` contains all the interpolations.

If we take an example above:

```js
const string = `something ${1 + 2 + 3}`
```

`literals` is an array with two items. The first is `something`, the string until the first interpolation, and the second is an empty string, the space between the end of the first interpolation (we only have one) and the end of the string.

`expressions` in this case is an array with a single item, `6`.

A more complex example is:

```js
const string = `something
another ${'x'}
new line ${1 + 2 + 3}
test`
```

in this case `literals` is an array where the first item is:

```js
;`something
another `
```

the second is:

```js
;`
new line `
```

and the third is:

```js
;`
test`
```

`expressions` in this case is an array with two items, `x` and `6`.

The function that is passed those values can do anything with them, and this is the power of this kind feature.

The most simple example is replicating what the string interpolation does, by joining `literals` and `expressions`:

```js
const interpolated = interpolate`I paid ${10}€`
```

and this is how `interpolate` works:

```js
function interpolate(literals, ...expressions) {
  let string = ``
  for (const [i, val] of expressions.entries()) {
    string += literals[i] + val
  }
  string += literals[literals.length - 1]
  return string
}
```