While browsing through some declaration files on DefinitelyTyped, I have noticed the following pattern:
declare function domready(callback: () => any) : void;
export = domready;
export as namespace domready;
I am familiar with the first two lines - declaring a function and exporting it as the module object. However, I am unsure about the purpose of the last line. Is it necessary in my declaration file? What exactly does it do?
Explaining the concept of a UMD module declaration.
If you're not already familiar, UMD stands for Universal Module Definition, which is a design pattern that allows a JavaScript library to function both as a global variable and as a modular component based on the environment it's used in.
Many libraries are structured using the UMD pattern. For instance, you might use the library like this in the browser:
moment.parse("12-31-2017")
But in node.js, your code may look more like this:
const m = require("moment");
m.parse("12-31-2017")
When you declare an export such as export function parse, TypeScript recognizes it as defining a module similar to the second example above. However, if the library being defined also creates a global variable, you can specify that global variable using UMD syntax as shown below:
export as namespace the_global_identifier;
Some UMD libraries always create a global variable, while others require import statements when used within an environment supporting module systems. Since libraries often lack clear documentation about their behavior, authors of declaration files may not know what to expect, resulting in TypeScript restricting access to the global identifier only if the referencing file is not a module.
To determine if a library follows the UMD pattern, look for patterns like the following:
(function (global, factory) {
typeof exports === 'object' && typeof module !== 'undefined' ? factory(exports) :
typeof define === 'function' && define.amd ? define(['exports'], factory) :
(factory((global.mymodule = global.mymodule || {})));
}(this, function (exports) { 'use strict';
function myFunction() {
console.log('hello world');
}
}));
This structure typically indicates a UMD module: extensive use of typeof checks for "exports," "module," and "define," followed by assignment to an "exports" parameter within a large function body. If this pattern is absent in the library code, it is likely not a UMD module, and an export as namespace declaration should not be included.
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