VSCode simple project specific extention. Place the files in the .vscode/extensions/<extention name> folder.

.vscode_launch.json

// A launch configuration that launches the extension inside a new window
// Use IntelliSense to learn about possible attributes.
// Hover to view descriptions of existing attributes.
// For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
{
	"version": "0.2.0",
	"configurations": [
		{
			"name": "Run Extension",
			"type": "extensionHost",
			"request": "launch",
			"args": [
				"--extensionDevelopmentPath=${workspaceFolder}"
			]
		}
	]
}

eslint.config.mjs

import globals from "globals";

export default [{
    files: ["**/*.js"],
    languageOptions: {
        globals: {
            ...globals.commonjs,
            ...globals.node,
        },

        ecmaVersion: 2022,
        sourceType: "module",
    },

    rules: {
        "no-const-assign": "warn",
        "no-this-before-super": "warn",
        "no-undef": "warn",
        "no-unreachable": "warn",
        "no-unused-vars": "warn",
        "constructor-super": "warn",
        "valid-typeof": "warn",
    },
}];

extension.js

const vscode = require('vscode');

/**
 * @param {vscode.ExtensionContext} context
 */
function activate(context) {

    const disposable = vscode.commands.registerCommand('example-autocomplete.helloWorld', function () {
        vscode.window.showInformationMessage('Hello world');
    });

    context.subscriptions.push(disposable);

    const provider = vscode.languages.registerCompletionItemProvider(
        { scheme: "file", language: "javascript" },
        {   
            provideCompletionItems(document, position) {
                let currentLine = document.lineAt(position.line).text;
                let firstWord = currentLine.trim().split(' ')[0].toUpperCase();

                return [
                    new vscode.CompletionItem("myCustomSnippet", vscode.CompletionItemKind.Snippet),
                    new vscode.CompletionItem("myFunction", vscode.CompletionItemKind.Function),
                    new vscode.CompletionItem(currentLine, vscode.CompletionItemKind.Enum),
                ];
            },
        },
        "'", "~" // trigger characters
    );
    context.subscriptions.push(provider);
}

function deactivate() { }

module.exports = {
    activate,
    deactivate
};

jsconfig.json

{
	"compilerOptions": {
		"module": "Node16",
		"target": "ES2022",
		"checkJs": true,  /* Typecheck .js files. */
		"lib": [
			"ES2022"
		]
	},
	"exclude": [
		"node_modules"
	]
}

package.json

{
  "name": "example-autocomplete",
  "displayName": "example-autocomplete",
  "description": "",
  "version": "0.0.1",
  "engines": {
    "vscode": "^1.104.0"
  },
  "categories": [
    "Other"
  ],
  "activationEvents": [],
  "main": "./extension.js",
  "contributes": {
    "commands": [{
      "command": "example-autocomplete.helloWorld",
      "title": "Hello World"
    }]
  },
  "scripts": {
    "lint": "eslint .",
    "pretest": "npm run lint"
  },
  "devDependencies": {
    "@types/vscode": "^1.104.0",
    "@types/mocha": "^10.0.10",
    "@types/node": "22.x",
    "eslint": "^9.34.0"
  }
}