What happens to this when a method is detached from its object?

Direct answer

Detaching a regular method removes the object-method call site; when the detached function is called plainly, it no longer receives the original object as this.

detached-method-this.js

"use strict";

const user = { name: "Ada", getName() { return this.name; } };
const getName = user.getName;

console.log(user.getName());
getName();

Output

Ada
TypeError: Cannot read properties of undefined (reading 'name')

Given const user={name:'Ada', getName(){ return this.name; }}; const getName=user.getName;, the expressions user.getName() and getName() have different call sites.

user.getName() uses user as the receiver and returns 'Ada'. In strict-mode code, a plain getName() call gives the function undefined as this, so trying to read this.name throws. Behavior involving a global object in non-strict classic scripts should not be relied on.

Preserve the receiver with const getName = user.getName.bind(user);.
Or wrap the invocation: const getName = () => user.getName();.
This issue commonly appears when methods are passed as callbacks without binding.