Does importing only a single function also runs other functions in that file?

I have this code in the file bmiCalculator.ts

import { isNotNumber } from "./utils";

export default function calculateBmi(height: number, weight: number) {

const bmi = weight / Math.pow(height / 100, 2);

if (bmi < 18.5) {

return "Underweight";

} else if (bmi >= 18.5 && bmi < 25) {

return "Normal (healthy weight)";

} else {

return "Overweight";



const parseArguments = (args: string[]) => {

if (args.length < 4) throw new Error("Not Enough Arguments");

if (args.length > 4) throw new Error("Too many arguments");

if (!isNotNumber(args[2]) && !isNotNumber(args[3])) {

return {

height: Number(args[2]),

weight: Number(args[3]),


} else {

throw new Error("Arguments are not of the correct type");



const { height, weight } = parseArguments(process.argv);

console.log(calculateBmi(height, weight));

And also this code in the file index.ts

import express from "express";

import calculateBmi from "./bmiCalculator";

const app = express();

app.get("/hello", (_req, res) => {

res.send("Hello Full Stack!");


app.get("/bmi", (req, res) => {

const height = Number(req.query.height);

const weight = Number(req.query.weight);

const bmi = calculateBmi(height, weight);

res.send({ weight, height, bmi });


const PORT = 3002;

app.listen(PORT, () => console.log(`server running on port ${PORT}`));

I am having this error while running the express server.

Error: Not Enough Arguments

But why?? I am not running the parseArgument function. I am only importing the calculateBmi function. So why this error?

Right now, I can only run the code by commenting out the non-exported code in bmiCalculator.ts

>Solution :

Yes, as soon as you import a module (here the module is just a file), it is executed.

If the module contains only exported objects, then there is no side effect.

If the module contains some root level expressions (like const { height, weight } = parseArguments(process.argv); in your case), they are executed. In particular, you should see your next console.log printing.

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