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Hackernoon logoWhat Is Tagged Template Literal In Javascript by@greenroots

What Is Tagged Template Literal In Javascript

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@greenrootsTAPAS ADHIKARY

UI/UX | Blogger @blog.greenroots.info| Speaker

Before ES6(ECMAScript 2015), we have used single quotes('...') and double quotes("...") to wrap string literals. A simple example is,

var msg = "Hello, I'm Joe and my favorite color is purple";

There were limitations when we had to concatenate multiple strings and the string literal has dynamic values. The readability of these concatenations used to be a challenge as well.

var frag1 = "Hello, I'm";
var val1= "Joe";
var frag2 = "and my favorite color is";
var val2 = "purple";

var msg = frag1 + ' ' + val1 + ' ' + frag2 + ' ' + val2;

Do you see another problem? If someone only reading the line where the concatenation takes place, he/she doesn't have much idea of the resultant string.

With ES6 we have gotĀ template literalsĀ which are string literals that allow embedding expressions. Template literals are enclosed by the backtickĀ (` `)Ā characters instead of single or double-quotes.

Template literalsĀ can contain placeholders that are specified by the dollar sign($) and curly braces(${expression}). The above example can be written with template literals as,

const name = 'Joe';
const color = 'purple';

const message = `Hello, I'm ${name} and my favorite color is ${color}`;
console.log(message);

Output,

Hello, I'm Joe and my favorite color is purple

This is much better and favorable for developers to use.

What is the Tagged Template Literal?

AĀ Tagged Template LiteralĀ is usually a function that precedes aĀ template literalĀ to help you in manipulating the output. It's fine if you find it confusing. We will understand it in a few simple steps.

Let us take an example of this template literal again,

`Hello, I'm ${name} and my favorite color is ${color}`

We want to manipulate the output such that, it returns a string like the below when we pass the name as,Ā JoeĀ and color as,Ā green.

Hello Joe, Have a Nice Day! We know your favorite color is green

How about, displaying this message in the color that is passed as an expression to the template literal? Like this when the color value isĀ green,

Welcome Tag function

Let us first create aĀ tagĀ function. This is a regular JavaScript function that should return a value as per your needs. This return value is usually a manipulated output based on the template literal strings and expressions.

function introduce() {                
    return 'introduce...';
}

Next, We mention theĀ tagĀ function before the template literal so that, the tag function gets associated with it.

const name = 'Joe';
const color = 'green';

const message = introduce`Hello, I'm ${name} and my favorite color is ${color}`;

Please note the tag functionĀ introduceĀ before the template literal.

Tag function takes arguments

The first argument of aĀ tag functionĀ contains an array of string values. The remaining arguments are related to the expressions.

function introduce(strings, arg0, arg1) {
  console.log('strings', strings);
  console.log('arg0', arg0);
  console.log('arg1', arg1);

  return 'introduce...';
}

const name = 'Joe';
const color = 'purple';

const message = introduce`Hello, I'm ${name} and ${color} is my favorite color!`;

The argumentĀ stringsĀ is an array of all the strings in the template literals and bothĀ arg0Ā andĀ arg1Ā represent theĀ nameĀ andĀ colorĀ values here.

Output,

Passing the expressions as individual arguments is not so great. Think about it, if there are 10 expressions in a template literal. We can make use ofĀ JavaScript rest operator(...values)Ā to collect the arguments as an array.

function introduce(strings, ...values) {
  console.log('strings', strings);
  console.log('values', values);

  return 'introduce...';
}

const name = 'Joe';
const color = 'purple';

const message = introduce`Hello, I'm ${name} and ${color} is my favorite color!`;

In this case, bothĀ stringsĀ andĀ valuesĀ are arrays. TheĀ stringsĀ argument contains all the strings where theĀ valuesĀ argument contains all the expression values.

Output,

Now, we can do everything possible with these strings and expression values to manipulate them.

The desired output

To get the desired output after the string manipulation, we will write a small logic inside theĀ introduceĀ function.

unction introduce(strings, ...values) {                                                        
   let msg = 
     `<span style="color:${values[1]}">
           Hello ${values[0]}, Have a Nice Day! We know your favorite color is <u>${values[1]}</u>
      </span>`;

   return msg;
}

const name = 'Joe';
const color = 'green';

const message = introduce`Hello, I'm ${name} and ${color} is my favorite color!`;

console.log(message);

We create a new template literal using the expression values and wrap it with theĀ spanĀ element. Please notice, we have added a style to the span element to color the text as well.

Output,

<span style="color:green">
    Hello Joe, Have a Nice Day! We know your favorite color is <u>green</u>
</span>

Now, if you use the above output to add asĀ innerHTMLĀ you can render it in the browser.

document.body.innerHTML = message;

Output,

The text color will change as and when you change the color variable value in your code.

Did you know šŸ’…?

If you are familiar withĀ reactjs, you probably know about theĀ styled-component. But, did you know,Ā styled-componentsĀ are created usingĀ tagged template literals?

Yes true. Notice the syntax of a button created with the styled-component,

const Button = styled.button`
  background-color: papayawhip;
  border-radius: 3px;
  color: palevioletred;
`

Does it look familiar to the tagged template literal we learned? Read this awesome articleĀ The magic behind šŸ’… styled-componentsĀ to know more about it.

Conclusion

Tagged Template LiteralsĀ are powerful and the usage will vary from one application to another. At the same time, if you were new to it before reading the article, lookout for opportunities to use it.

I have updated theĀ js-tips-tricksĀ project in GitHub with code examples. You may want to have a look.

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If it was useful to you, please Like/Share so that, it reaches others as well. You can @ me on Twitter (@tapasadhikary) with comments, or feel free to follow me.

Previously published at https://blog.greenroots.info/what-exactly-is-javascript-tagged-template-literal-ckg6hyekf000n8bs1hz9udvzc

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