Importing node_modules in Angular2 using TypeScript

My Angular2 app started off as a simple 'hello world' project. However, I made the questionable decision to use different directory structures for my development environment and the final deployment folder on my spring backend.

This difference caused an issue with TypeScript imports, leading to 404 errors when trying to open the app in the browser. The problematic line was:

import {Component, View} from 'angular2/core';

To resolve the issue, I added back the /app folder and changed my import statements like so:

import {Component, View} from '../node_modules/angular2/core';

Unfortunately, this change resulted in some strange behavior. Specifying ../node_modules in the library paths triggered the loading of ALL Angular2 files individually through ajax calls, even though the script tag was correctly included in the HTML:

<script src="/node_modules/angular2/bundles/angular2.dev.js"></script>

Upon discovering the cause of the issue, I reverted the import statement back to its original form:

import {Component, View} from 'angular2/core';

With this adjustment, Angular2 loaded seamlessly from the script tag without any additional ajax calls for file retrieval.

I would appreciate it if someone could shed light on why specifying a detailed path in the import statement had such a significant impact on the loading mechanism of Angular2. It seemed like normal behavior, but I am still puzzled by how it all works.

Answer №1

When working with TypeScript, the import rules closely mirror those of node.js. If an import starts with a dot:

import {Something} from './some/path';

It is considered a relative path from the file that contains the import statement. However, if it is an absolute path:

import {Component} from 'angular2/core';

It is recognized as an external module. TypeScript will search for a package.json file by moving up the directory tree, then navigate to the node_modules folder, locate a folder matching the import name, check the module's package.json for the main .d.ts or .ts file, and load that. Alternatively, it can also look for a file with the same name as specified in the import, or an index.d.ts or index.ts file.

Although this may seem intricate, especially with exceptions, familiarity with node.js should make it more manageable.

Note that setting a TypeScript compiler option in tsconfig.json is essential for typing resolutions to function as described:

"moduleResolution": "node"

The second part of your query pertains to loading mechanisms without relying on ajax calls. This functionality is enabled by System.js. By importing a bundle in the index.html script tag, System registers the angular2 bundle, facilitating proper assignment of references. While delving deeper into this topic is recommended, extensive information can be found in the README of systemjs, or systemjs-builder.

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