Typescript - Conditional imports

Question

Typescript - Conditional imports

When working with the moment-timezone module, one issue that arises is receiving a warning if it is included multiple times. In my specific case, I have a module that necessitates the use of this timezone functionality. Since I am unsure whether or not the user will manually set the timezone themselves, my approach is as follows:

if (moment.tz === undefined) {
    require('moment-timezone');
    moment.tz.setDefault('America/Los_Angeles');
}

While this solution works seamlessly in standard JavaScript, I recently encountered an obstacle when transitioning to TypeScript. The error message produced states: Cannot find name 'require'.

I attempted to address this by adjusting the line to read

import momentTimezone = require('moment-timezone');

, but then confronted another error:

An import declaration can only be used in a namespace or module.

How should I proceed in resolving this dilemma?

javascript typescript

Answer 1

Answer №1

In TypeScript, the import foo = require ('foo') statement serves a dual purpose. It brings in both the type definitions into the declaration space and the actual module into the variable space. If you only use the imported module for typings without actually accessing it in your code, it will be stripped out at runtime. However, if you interact with the imported module by calling its methods or assigning it to variables, a runtime import will occur. Understanding this behavior is crucial as it can prevent unexpected surprises.

If you wish to load the type definitions and conditionally import the module based on some criteria, you can combine TypeScript's import syntax with regular require statements like so:

import foo = require('foo');

export function loadFoo() {
// This approach lazily loads `foo` and uses the original module solely for typing purposes
    var _foo: typeof foo = require('foo');
    // Now you can utilize `_foo` as a variable instead of directly using `foo`.
}

Answer 2

In TypeScript, the import foo = require ('foo') statement serves a dual purpose. It brings in both the type definitions into the declaration space and the actual module into the variable space. If you only use the imported module for typings without actually accessing it in your code, it will be stripped out at runtime. However, if you interact with the imported module by calling its methods or assigning it to variables, a runtime import will occur. Understanding this behavior is crucial as it can prevent unexpected surprises.

If you wish to load the type definitions and conditionally import the module based on some criteria, you can combine TypeScript's import syntax with regular require statements like so:

import foo = require('foo');

export function loadFoo() {
// This approach lazily loads `foo` and uses the original module solely for typing purposes
    var _foo: typeof foo = require('foo');
    // Now you can utilize `_foo` as a variable instead of directly using `foo`.
}

Typescript - Conditional imports

Answer №1

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