How can I fix WebStorm warning "Unresolved function or method" for "require" (Firefox Add-on SDK)

Question

Do you mean that require() is not resolved? You need to either add require.js to your project or enable Node.js Globals predefined library in Settings/Languages and Frameworks/JavaScript/Libraries.

(Edited settings path by @yurik)

In WebStorm 2016.x-2017.x: make sure that the Node.js Core library is enabled in Settings (Preferences) | Languages & Frameworks | Node.js and NPM

In IntelliJ 2018.3.2+ go to Settings (Preferences) | Languages & Frameworks | Node.js and NPM and enable Coding assistance for Node.js

Webstorm 11 and 2016.2.3

Enable Node.js Core library in Webstorm settings.

After spending an hour trying to get this to work using all solutions found online, this finally did the trick!

File -> Invalidate Caches/Restart...

In PHPStorm, it's a bit easier: you can just search for NPM in settings or:

File > Settings > Language & Frameworks > Node.js and NPM

Then click the enable button (apparently in new versions, it is called "Coding assistance for Node").

Working with Intellj 2016, Angular2, and Typescript... the only thing that worked for me was to get the Typescript Definitions for NodeJS

Get node.d.ts from DefinitelyTyped on GitHub

Or just run:

npm install @types/node --save-dev

Then in tsconfig.json, include

"types": [
     "node"
  ]

In WebStorm 2018.3 a bit different interface

On WebStorm 2016.3

1. Press ALT+F12 (open terminal)

2. Run this command:

   npm install require.js