Understanding Tagged Template Literals in JavaScript

What are Tagged Template Literals?

A tagged template literal involves a template literal prefixed with a function, called a tag. This function can process and manipulate the literal's content. Here's a simple example:

function tag(strings, ...values) {
    console.log(strings);
    console.log(values);
    return 'Processed string';
}

const name = 'Alice';
const greeting = tag`Hello, ${name}! How are you?`;
console.log(greeting);

Use Cases for Tagged Template Literals

1. Internationalization (i18n)

Tagged template literals can dynamically translate strings based on the user’s locale. Here’s an example using Japanese:

function i18n(strings, ...values) {
    const translations = {
        'Hello, ': 'こんにちは、',
        '! How are you?': '！元気ですか？',
    };

    return strings.reduce((result, str, i) => result + translations[str] + (values[i] || ''), '');
}

const name = 'アリス';
const greeting = i18n`Hello, ${name}! How are you?`;
console.log(greeting); // Output: "こんにちは、アリス！元気ですか？"

2. Custom String Formatting

They can also implement custom formatting logic, such as escaping HTML.

function escapeHTML(strings, ...values) {
    const escape = (str) => str.replace(/&/g, '&amp;').replace(/</g, '&lt;').replace(/>/g, '&gt;');
    return strings.reduce((result, str, i) => result + str + escape(values[i] || ''), '');
}

const userInput = '<script>alert("XSS")</script>';
const sanitized = escapeHTML`User input: ${userInput}`;
console.log(sanitized); // Output: "User input: &lt;script&gt;alert(&quot;XSS&quot;)&lt;/script&gt;"

Conclusion

Tagged template literals provide a versatile tool for dynamic string manipulation in JavaScript. They can simplify tasks like internationalization and custom string formatting, leading to more expressive and maintainable code.