JavaScript での文字列補間。

Introduction to Template Literals.

String manipulation is a very common task in programming, especially when building interactive web applications. If you've ever spent time working with JavaScript, then you've likely had to put some variables into strings.
In older versions of JavaScript, this meant using the + operator to join strings together through a process called string concatenation. However, with the introduction of template literals in the JavaScript ES6(2015) update. We now have a cleaner way to insert variables into strings, called string interpolation.

What are Template Literals?

Template literals allow us to manipulate strings easier. They are enclosed in backticks (`) rather than (') or ("), and they support string interpolation by using the (${}) syntax to place variables, or function calls directly into a string.

Here’s an example of how template literals simplify string interpolation.

const name = "John"
const age = 24

// Old method using concatenation
const greeting = "Hello, " + name + "! You are " + age + " years old."

// New method using template literals
const greetingWithTemplateLiteral = `Hello, ${name}! You are ${age} years old.`

console.log(greetingWithTemplateLiteral) // Outputs: Hello, John! You are 24 years old.

Benefits of Using Template Literals

1. Improved Readability

When using string concatenation, it was easy to get lost in a bunch of + signs, especially when working with longer strings. Template literals avoid this by letting you write strings in a way that is easier to follow.

const product = "laptop"
const price = 1400

console.log(`The price of the ${product} is $${price}`)
// Outputs: The price of the laptop is $1400

2. Multi-line Strings

Before template literals, you had to use escape characters like \n to make multi-line strings. Now you can write them inside backticks(`).

// Old method
const multiLineString1 = "This is the first line" + "\n" + "This is the second line" + "\n" + "This is the third line"

// New method
const multiLineString2 = `This is the first line
This is the second line
This is the third line`

console.log(multiLineString1)
console.log(multiLineString2)
/* Both output:
This is the first line
This is the second line
This is the third line
*/

3. Expression Evaluation

You can also perform calculations, call functions, or manipulate data inside strings.

const a = 1
const b = 10

console.log(`The sum of a and b is ${a + b}`) 
// Outputs: The sum of a and b is 11

const upperCaseName = (name) => name.toUpperCase()
console.log(`Your name in uppercase is ${upperCaseName("John")}`)
// Outputs: Your name in uppercase is JOHN

Common Use Cases in JavaScript

1. HTML Generation

Instead of building HTML strings with concatenation, you can put variables directly into the string with interpolation.

const name = "John"
const htmlContent = `
  <h1 id="Hello-name">Hello, ${name}!</h1>
  <p>Welcome to the site.</p>
`

2. Logging Messages

You can also insert variables directly into log messages without the need for concatenation.

const user = "John"
const action = "logged in"

console.log(`User ${user} just ${action}`)
// Outputs: User John just logged in

3. Creating URLs

Template literals make it easier to construct URLs as well.

const userId = 123
const apiUrl = `https://api.example.com/user/${userId}/details`

console.log(apiUrl)
// Outputs: https://api.example.com/user/123/details

4. Conditional Logic

Another great use case is Conditional logic. With template literals you can give strings simple conditions using the ternary operator (? :), which is a shorthand for an if-else condition.
Logical operators like && (and) or || (or) can also be used to add conditional parts to a string. This removes the need for extra if-else statements or the need for concatenation.

const isMember = true
console.log(`User is ${isMember ? 'a member' : 'not a member'}`) 
// Outputs: User is a member

You can also add more complex expressions inside template literals.

/* In this example, the condition age >= 18 is evaluated
the result is either “an adult” or “a minor” based on the value of age*/
const age = 24

const message = `You are ${age >= 18 ? 'an adult' : 'a minor'}`

console.log(message) 
// Outputs: You are an adult

/*In this, if isLoggedIn is true and username exists
username is displayed or else, it defaults to “Guest” */
const isLoggedIn = true
const username = "John"

const greeting = `Welcome ${isLoggedIn && username ? username : 'Guest'}`

console.log(greeting)
// Outputs: Welcome John

Conclusion

Template literals in JavaScript offer a cleaner, and more efficient way to handle string interpolation. Between building web content, logging messages, or creating more readable code, this method provides the flexibility you need.

Next time your juggling with variables and strings, try using template literals. You'll quickly see why it's my go-to method for working with JavaScript.

Resources

1. MDN Web Docs - Template Literals
2. GitHub - Phase 1 Review Strings Lab
3. W3 Schools - JavaScript Template Strings

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