Unable to write a polyfill for call function [duplicate]

This is my polyfill for call method

Function.prototype.mycall = (obj,...args)=>{
    if(typeof this!="function") throw new Error("mycall can only be used on a function")
    obj.myFunc = this
    obj.myFunc(...args)
    delete obj.myFunc
}

And this is my whole script including the polyfill

obj = {
    name: "sai",
    age: 25
}

sample = ()=>{
    console.log(`I am ${this.name} and is ${this.age} years old`)
}

Function.prototype.mycall = (obj,...args)=>{
    if(typeof this!="function") throw new Error("mycall can only be used on a function")
    obj.myFunc = this
    obj.myFunc(...args)
    delete obj.myFunc
}


sample.mycall(obj)

But when I look at my console, I am getting this error

task.js:12 Uncaught Error: mycall can only be used on a function
    at Function.mycall (task.js:12:39)
    at task.js:22:8
Function.mycall @ task.js:12
(anonymous) @ task.js:22

Can anyone please help me on why I am getting this error?