JavaScript default Keyword: A Comprehensive Guide
The default keyword in JavaScript plays a crucial role in ES6 modules, particularly when exporting and importing values. It allows you to designate a primary export from a module, which simplifies importing and usage. This guide provides a thorough explanation of how to use the default keyword, supported by clear syntax, examples, and practical applications.
What is the default Keyword?
In the context of JavaScript modules, the default keyword is used to define a default export. A module can only have one default export. This export is treated as the main value that the module intends to provide. When importing from such a module, you can import the default value without specifying a name, simplifying the import process.
Purpose of the default Keyword
The primary purpose of the default keyword is to:
- Simplify Module Imports: Allows for simpler imports of a primary value from a module.
- Establish a Main Export: Clearly defines the main entity a module is designed to export.
- Enhance Code Readability: Makes the intention of modules clearer and code easier to follow.
- Facilitate Common Usage Patterns: Supports widely used practices in modern JavaScript development.
Syntax of default Exports
export default
The export default syntax is used to export a value as the default export of a module. It is typically used in a module’s file.
// module.js
export default function myFunction() {
console.log("This is the default function.");
}
Note: You can export a variety of types as the default export, including functions, objects, classes, and primitive values. 💡
Importing Default Exports
import defaultExport from 'module.js';
The syntax for importing a default export is simple and straightforward. You can import the default export using a chosen name, without curly braces.
// main.js
import myFunction from './module.js';
myFunction(); // Output: This is the default function.
import { somethingElse, default as renamedDefaultExport } from 'module.js';
You can import other named exports along with a renamed default export. You can also rename a default export using as.
// module.js
export default function myFunction() {
console.log("This is the default function.");
}
export const myVariable = "Hello";
// main.js
import renamedDefaultExport, { myVariable } from './module.js';
renamedDefaultExport(); // Output: This is the default function.
console.log(myVariable); // Output: Hello
Practical Examples of default
Let’s explore some practical examples to understand how to use the default keyword effectively.
Default Exporting a Function
This is the most common use case of the default keyword, where a module exports a single function.
module.js
// module.js
export default function greet(name) {
return `Hello, ${name}!`;
}
main.js
// main.js
import greet from './module.js';
console.log(greet("Alice")); // Output: Hello, Alice!
Default Exporting an Object
A module can also export a default object, often used for configuration or a set of related properties.
config.js
// config.js
export default {
apiUrl: 'https://api.example.com',
theme: 'dark',
debugMode: true
};
app.js
// app.js
import config from './config.js';
console.log(config.apiUrl); // Output: https://api.example.com
console.log(config.debugMode); // Output: true
Default Exporting a Class
Classes can also be default exported, useful for creating reusable components.
component.js
// component.js
export default class Button {
constructor(text) {
this.text = text;
}
render() {
return `<button>${this.text}</button>`;
}
}
index.html
<!DOCTYPE html>
<html>
<head>
<title>Default Class Export</title>
</head>
<body>
<div id="container_default"></div>
<script type="module" src="app.js">
</script>
</body>
</html>
app.js
// app.js
import Button from './component.js';
const myButton = new Button('Click Me');
const container_default = document.getElementById('container_default');
container_default.innerHTML = myButton.render();
Output: A button with “Click Me” text in a HTML page.
Default Exporting a Variable
Even simple primitive values or variables can be the default export of a module.
constants.js
// constants.js
export default 42;
main.js
// main.js
import answer from './constants.js';
console.log(answer); // Output: 42
Combining Default and Named Exports
Modules can have both default and named exports. In such cases, you can import them together.
utils.js
// utils.js
export default function formatText(text) {
return text.toUpperCase();
}
export function reverseText(text) {
return text.split("").reverse().join("");
}
app.js
// app.js
import formatText, { reverseText } from './utils.js';
console.log(formatText("hello")); // Output: HELLO
console.log(reverseText("world")); // Output: dlrow
Exporting with default as Keyword
You can also use the default as keyword to export a variable or a function as a default export.
helper.js
// helper.js
function calculateSum(a, b) {
return a + b;
}
export { calculateSum as default };
main.js
// main.js
import calculateSum from './helper.js';
console.log(calculateSum(5, 3)); // Output: 8
Benefits of Using the default Keyword
- Simplified Importing: Easier syntax for importing the primary export of a module.
- Improved Code Clarity: Makes it clear what the main export of a module is.
- Consistent Module Patterns: Encourages consistent practices in modern JavaScript development.
- Flexibility: Supports exporting various types like functions, objects, classes, and values.
When to Use default Keyword
- Single Main Functionality: When a module primarily exports a single function or class.
- Configuration Objects: For exporting an object that holds configuration data.
- Reusable Components: When creating reusable UI components with classes.
- Primary Constants: If a module provides a primary constant value.
When Not to Use default Keyword
- Multiple Exports: If a module exports many independent items, use named exports instead.
- Clear Naming: When specific naming is crucial for exports, avoid default exports.
- Avoiding Confusion: To prevent ambiguity when many exports are needed from the same module, prefer named exports.
Conclusion
The default keyword is a vital part of modern JavaScript module syntax, providing a straightforward way to manage primary exports from modules. Understanding how to properly use default exports and imports is crucial for writing modular and maintainable JavaScript code. By following the examples and guidelines outlined in this article, you can effectively use the default keyword to write cleaner and more readable code.
