VSCode does not show Code Helper for imports from another file

Question

I'm new to Node.JS and VSCode.

I'm coding Firestore Cloud Function. I want to use a constant file to help me make sure I don't make type-errors.

I have followed the suggestion here. But the VSCode code helper doesn't show the constants I have defined there.

In constants.js:

function define(name, value) {
    Object.defineProperty(exports, name, {
        value:      value,
        enumerable: true
    });
}

define('kFieldName', 'name');
define('kFieldAge', 'age'); 

Then in index.js:

var constants = require("./constants");

exports.createUserDocuments = functions
    .runWith({ memory: "128MB", timeoutSeconds: 30 })
    .auth.user().onCreate((user) => {

    // As I type constants.kFieldName here code helper does not show the corresponding definitions

});

What I hope to see is as I type the keyword such as constants.kField then the Code Helper should show suggestions kFieldName and kFieldAge just like in Android Studio.

How do I make VSCode's code helper to show the definitions I wrote in constants.js? Is it because I miss something in regards to exports and importing the files?

Thank you!