How VSCode injects "vscode" engine into the extensions?

Question

When developing extensions for VSCode. We see this import:

import * as vscode from 'vscode';

and in package.json, we have

"engines": {
  "vscode": "*"
}

Nowhere in the dependencies we have 'vscode'. But, looks like it is available for extension. Any explanation would be appreciated.

Answer 1

Imports are resolved by the host environment, in this case VSCode's possibly-modified version of Electron. So when it sees a request for the vscode module, it provides it (internally) rather than looking for an external dependency.

FWIW, a defacto standard is emerging that "raw" module names, like 'vscode', tend to be provided directly by the host environment whereas ones with paths ('./foo') are external. (That's why the src on script type="module" tags is required to have a path, at least for now.)

Answer 2

The "engines" section in package.json is not related with module import system.
It's for some native module to know howto compile when npm install.
And can check engine version. eg: you can set engines: {node: >=8}, then node v7 will deny to run your code, but that's not enforce.

VS Code is using vscode-loader as module loader, it's very like require.js, but has many other function for vscode.
The "global" function "require" you called, is override by vscode-loader, not node's native "require".
Same with any other module loader system, vscode-loader allows you to modify the "require" function.

vscode is changing so fast, you can do a simple search with nodeRequire('module').

Currentlly, related code is in src/vs/workbench/api/node/extHost.api.impl.ts file:

const node_module = <any>require.__$__nodeRequire('module');
const original = node_module._load;
node_module._load = function load(request: string, parent: any, isMain: any) {
    if (request !== 'vscode') {
        return original.apply(this, arguments);
    }
    .....
    .....
    // and finally, return apiImpl, the "vscode" object
}

require() will call module._load(), but this module._load is already overrided by vscode-loader.
You can also override it again like this.
That is called "Monkey Patch".