this is undefined inside prototype function

Question

I have below two functions in two seperate files:

//homeFunction.js

import secondaryFunc from "./secondaryFunction.js";

const secFunc = new secondaryFunc(3);

secFunc.sayHello(5);

The another file is secondaryFunction.js:

function secondaryFunc(val){
    this.val = val;
}

secondaryFunc.prototype.sayHello = (val1)=>{
    console.log(this);
    console.log("Hello");
}

export default secondaryFunc;

Here, when I execute homeFunction, this is undefined inside sayHello function, which I don't understand because I have already created a reference to the secondaryFunc, which has sayHello as a property, so this should not be undefined inside sayHello. Please help me to understand this anomaly.

Thanks!