Passing const variables to functions in TypeScript

Question

I am learning TypeScript and I am a bit confused by the way it handles const variables. Let's say I define an interface for a number as:

interface MyNumber {
    value: number;
}

and created a const MyNumber

const myNumber: MyNumber = { value: 42 };

I read in the TypeScript documentation that const did not prevent me from modifying the internal state of the object (provided the property is not readonly). However, I did not expect this to compile:

function setToFive(num: MyNumber) {
    num = {
        value: 5
    }
}

console.log(myNumber);
setToFive(myNumber);
console.log(myNumber);

This code prints 42 twice. It looks like the function performs a copy of my const variable and uses that copy within its scope. I find this a bit surprising. Is there a way to trigger a compile-time error instead?

you could mark the value field as readonly. That gives you a bit more of protection against accidental assignments. — Mario Vernari
– Mario Vernari, Commented Nov 12, 2019 at 15:04
Const declaration is not really as immutable as it should be. If a variable holds advanced type ( Object ) you can still change it's properties. Well, if you really want to achieve immutability, you have to use Object.freeze() — Alexander Gharibashvili
– Alexander Gharibashvili, Commented Nov 12, 2019 at 15:16

baklazan · Accepted Answer · 2022-12-01 11:08:12Z

4

You are reassigning the function parameter num to your new object. You cannot put const in function parameters.

What you are looking for is eslint or something similar. A tool to force best principles on your project with rule no param reassign.

edited Dec 1, 2022 at 11:08

answered Nov 12, 2019 at 14:59

baklazan

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1 Comment

Touloudou Over a year ago

That's exactly what I was looking for. Thank you.

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