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Import.lazy() method

Provides a way to improve process startup times by lazy-loading imported modules.

Signature:

static lazy(moduleName: string, require: (id: string) => unknown): any;

Parameters

Parameter Type Description
moduleName string  
require (id: string) => unknown  

Returns:

any

Remarks

This is a more structured wrapper for the import-lazy package. It enables you to replace an import like this:

import * as example from 'example'; // <-- 100ms load time

if (condition) {
  example.doSomething();
}

…with a pattern like this:

const example: typeof import('example') = Import.lazy('example', require);

if (condition) {
  example.doSomething(); // <-- 100ms load time occurs here, only if needed
}

The implementation relies on JavaScript’s Proxy feature to intercept access to object members. Thus it will only work correctly with certain types of module exports. If a particular export isn’t well behaved, you may need to find (or introduce) some other module in your dependency graph to apply the optimization to.

Usage guidelines:

  • Always specify types using typeof as shown above.

  • Never apply lazy-loading in a way that would convert the module’s type to any. Losing type safety seriously impacts the maintainability of the code base.

  • In cases where the non-runtime types are needed, import them separately using the Types suffix:

const example: typeof import('example') = Import.lazy('example', require);
import type * as exampleTypes from 'example';

  • If the imported module confusingly has the same name as its export, then use the Module suffix:
const exampleModule: typeof import('../../logic/Example') = Import.lazy(
  '../../logic/Example', require);
import type * as exampleTypes from '../../logic/Example';

  • If the exports cause a lot of awkwardness (e.g. too many expressions need to have exampleModule. inserted into them), or if some exports cannot be proxied (e.g. Import.lazy('example', require) returns a function signature), then do not lazy-load that module. Instead, apply lazy-loading to some other module which is better behaved.

  • It’s recommended to sort imports in a standard ordering:

// 1. external imports
import * as path from 'path';
import { Import, JsonFile, JsonObject } from '@rushstack/node-core-library';

// 2. local imports
import { LocalFile } from './path/LocalFile';

// 3. lazy-imports (which are technically variables, not imports)
const semver: typeof import('semver') = Import.lazy('semver', require);