Modules Dynamic Imports

Export and import statements that we covered in previous chapters are called "static". The syntax is very simple and strict.

First, we can't dynamically generate any parameters of import.

The module path must be a primitive string, can't be a function call. This won't work:

import ... from *!*getModuleName()*/!*; // Error, only from "string" is allowed

Second, we can't import conditionally or at run-time:

if(...) {
  import ...; // Error, not allowed!
}

{
  import ...; // Error, we can't put import in any block
}

That's because import/export aim to provide a backbone for the code structure. That's a good thing, as code structure can be analyzed, modules can be gathered and bundled into one file by special tools, unused exports can be removed ("tree-shaken"). That's possible only because the structure of imports/exports is simple and fixed.

But how can we import a module dynamically, on-demand?

The import() expression

The import(module) expression loads the module and returns a promise that resolves into a module object that contains all its exports. It can be called from any place in the code.

We can use it dynamically in any place of the code, for instance:

let modulePath = prompt("Which module to load?");

import(modulePath)
  .then(obj => <module object>)
  .catch(err => <loading error, e.g. if no such module>)

Or, we could use let module = await import(modulePath) if inside an async function.

For instance, if we have the following module say.js:

// 📁 say.js
export function hi() {
  alert(`Hello`);
}

export function bye() {
  alert(`Bye`);
}

...Then dynamic import can be like this:

let {hi, bye} = await import('./say.js');

hi();
bye();

Or, if say.js has the default export:

// 📁 say.js
export default function() {
  alert("Module loaded (export default)!");
}

...Then, in order to access it, we can use default property of the module object:

let obj = await import('./say.js');
let say = obj.default;
// or, in one line: let {default: say} = await import('./say.js');

say();

Here's the full example:

[codetabs src="say" current="index.html"]

Dynamic imports work in regular scripts, they don't require `script type="module"`.

Although `import()` looks like a function call, it's a special syntax that just happens to use parentheses (similar to `super()`).

So we can't copy `import` to a variable or use `call/apply` with it. It's not a function.