default

JavaScript default Keyword Syntax

The default keyword appears in switch statements and in default exports:

switch (<expression>) {
  [case <value>: <statements>] ...
  default: <statements>
}

export default <expression>;
export default function [<name>](<parameters>) { <body> }
export default class [<name>] { <body> }
export { <name> as default };

The function and class forms can leave out the name.

JavaScript default Keyword Examples

The default clause handles every value that no case matches:

> function statusText(code) {
…   switch (code) {
…     case 200:
…       return "OK";
…     case 404:
…       return "Not Found";
…     default:
…       return "Unknown status";
…   }
… }
undefined

> statusText(404)
'Not Found'
> statusText(500)
'Unknown status'

No case matches 500, so control jumps to the default clause.

Here’s a module with a default export, saved as greet.mjs:

export default function greet(name) {
  return "Hello, " + name + "!";
}

Another module, main.mjs, imports that function under a name of its choice:

import sayHello from "./greet.mjs";
import * as greetModule from "./greet.mjs";

console.log(sayHello("Ada"));
console.log(Object.keys(greetModule));

Running main.mjs prints the greeting and the namespace’s keys:

$ node main.mjs
Hello, Ada!
[ 'default' ]

The namespace object shows that the default export is stored under the name default.

JavaScript default Keyword Use Cases

• Handling values that no case clause matches in a switch statement
• Throwing an error for an unexpected value at the end of a switch
• Exporting the main function, class, or value of a module
• Letting importing modules pick their own name for the main export