What is the mechanism behind lazy module loading in typescript?

Exploring the concept of lazy loading in typescript, the author provides an example in this section of their book. They demonstrate it using the following piece of code:

import bar = require('bar');

    export function loadBar() {
        // Lazy loading `bar` and using the original module only for type annotation
        var _bar: typeof bar = require('bar');
        // Utilizing `_bar` as a variable instead of `bar`.
    }
    

The author explains that when using typescript, the type of 'bar' is only loaded upon first invocation of 'require', whereas the entire module is loaded on the second call when creating the 'bar' variable.

If you're curious about how exactly this mechanism works, could anyone provide a more elaborate example or delve deeper into what's happening behind the scenes?

Answer №1

Typescript has introduced a new feature in version 2.4 that allows for Dynamic Import Expressions, enabling lazy loading of modules.

Let's take a look at an example:

async function getZipFile(name: string, files: File[]): Promise<File> {
    const zipUtil = await import('./utils/create-zip-file');
    const zipContents = await zipUtil.getContentAsBlob(files);
    return new File(zipContents, name);
}

Under the hood, this functionality still utilizes require, as evidenced by the transpiled code here

Answer №2

The information can be found in the typescript handbook

By analyzing the usage of modules in the generated JavaScript code, the compiler determines when to emit require calls. If a module identifier is solely used in type annotations and not as an expression, no require call is included for that module.

In this particular case, the first instance of foo (the one without an underscore) is only used once in a type annotation with typeof, leading to the exclusion of the first require('foo') in the resulting javascript code. This can be observed by examining the generated .js file, showing only one call to require during runtime, specifically the 'second' one.

Upon invoking loadFoo(), the execution of require('foo') triggers the built-in node.js require() function, loading the foo module at runtime conventionally.

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