Why does Object() ensure correct this binding in a custom Function.call implementation?

Below is an explanation of why and how this technique works and an alternative implementation.

How the Technique Works

1. Converting thisArg to an Object
   When you write:

   JAVASCRIPTCopied
   thisArg = Object(thisArg);

   JavaScript ensures that if thisArg is a primitive value, it’s converted to its object wrapper (for example, a number becomes a Number object). This is important because only objects (or their wrappers) can hold properties. This corresponds to the ECMAScript specification’s steps for converting a value to an Object when a function expects an object receiver.

2. Setting the Function as a Method
   By creating a unique property key (using a Symbol) and assigning the function to it, you temporarily attach your function as a method of the thisArg object:

   JAVASCRIPTCopied
   const methodSymbol = Symbol();
   thisArg[methodSymbol] = this; // 'this' is your function.

   When you later invoke this property, the function is called as a method of thisArg, so its internal this is set to thisArg per standard method invocation rules.

3. Invocation and Cleanup
   After the function runs with the correct this, you remove the temporary property, ensuring no side effects remain on the passed object.

What’s Special About Object() Constructor

The Object() constructor (or its literal counterpart {}) here is used not to create a new object from a prototype but to ensure that thisArg is of an object type. If you pass a primitive (like a string or a number), wrapping it with Object() gives you an object that can have properties set on it. This is a necessary hack to simulate how call() works behind the scenes, ensuring that the function’s this always refers to an object.

An Alternative Implementation

A common reimplementation of Function.prototype.call might look like this:

JAVASCRIPTCopied
Function.prototype.myCall = function(thisArg, ...args) {
  // If thisArg is null or undefined, default to the global object:
  thisArg = thisArg == null ? globalThis : Object(thisArg);

  // Create a unique property to avoid collisions:
  const fnSymbol = Symbol();
  thisArg[fnSymbol] = this;

  // Invoke the function using the passed arguments:
  const result = thisArg[fnSymbol](...args);

  // Clean up the temporary property:
  delete thisArg[fnSymbol];

  return result;
};

// Example usage:
function greet(greeting, punctuation) {
  return greeting + ", " + this.name + punctuation;
}

const person = { name: "Alice" };
console.log(greet.myCall(person, "Hello", "!"));  // Output: "Hello, Alice!"

Summary

• Using Object(thisArg):
  This step guarantees that even if a primitive is given, it is wrapped into an object, and methods can be temporarily attached.

• Temporary Method Attachment:
  Adding the function as a method to this object ensures it is invoked with the desired this, leveraging the standard method invocation semantics of JavaScript.

• Cleanup:
  Removing the temporary property avoids side effects and retains the purity of your original object.

This approach is a well-known hack that demonstrates how JavaScript’s function invocation sets the context (this). It’s not something “magical” in the sense of hidden behavior but rather a creative use of the language’s core features.