Instance of returning false for cross-frame objects

Instance of Returning False for Cross-Frame Objects

In JavaScript, the instanceof operator is commonly used to check whether an object is an instance of a particular class or constructor. However, it can produce unexpected results, especially when working with objects from different execution contexts, such as different iframes or windows. One such issue occurs when instanceof returns false for an object that should logically be an instance of a particular class. This happens due to differences in prototype chains between different execution environments.

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1. Understanding the instanceof Operator

The instanceof operator works by checking if an object’s prototype chain includes the prototype property of the given constructor. It does this by traversing the prototype chain of the object.

Syntax

object instanceof Constructor

This returns true if object is an instance of Constructor, otherwise false.

How instanceof Works Internally

1. It retrieves the prototype property of the constructor function.
2. It walks up the prototype chain of the object, checking if it finds a match.
3. If a match is found, it returns true; otherwise, it returns false.

Example:

class Animal {}
const dog = new Animal();

console.log(dog instanceof Animal); // true

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2. The Issue with Cross-Frame Objects

When objects are created in different execution contexts (such as different iframes or browser windows), they may belong to different global objects (window or globalThis). Since each frame has its own JavaScript execution environment, constructor functions from different frames are treated as separate entities, even if they have the same name and structure.

Example: instanceof Failing Across Frames

const iframe = document.createElement("iframe");
document.body.appendChild(iframe);

const objFromIframe = new iframe.contentWindow.Array();
console.log(objFromIframe instanceof Array); // false ❌

Why Does This Happen?

• Each iframe has its own global object (window), which means Array in the iframe is different from Array in the main document.
• When you check objFromIframe instanceof Array, it fails because Array from the main window is not the same as Array from the iframe.
• The instanceof operator only checks against the current window’s prototype chain, so it doesn’t recognize constructors from another frame.

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3. Solutions to Fix the Issue

A. Using Object.getPrototypeOf()

Instead of relying on instanceof, you can check the prototype directly:

console.log(Object.getPrototypeOf(objFromIframe) === Array.prototype); // true ✅

This works because Object.getPrototypeOf(objFromIframe) correctly returns the prototype, regardless of the execution context.

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B. Using isPrototypeOf()

Another alternative is isPrototypeOf(), which explicitly checks if an object exists in the prototype chain:

console.log(Array.prototype.isPrototypeOf(objFromIframe)); // true ✅

This method is more reliable when working across frames because it doesn’t rely on constructors from different execution contexts.

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C. Using Object.prototype.toString.call()

You can use Object.prototype.toString.call() to get a reliable type string:

console.log(Object.prototype.toString.call(objFromIframe)); // "[object Array]"

This method works well for built-in objects but is not suitable for custom classes.

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D. Creating a Shared Reference

If you need to work with instances across frames, you can pass references explicitly:

const sharedArray = window.Array; // Reference from the main window
const obj = new sharedArray();

console.log(obj instanceof sharedArray); // true ✅

This ensures that objects are created using the same constructor reference.

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4. Key Takeaways

✅ Different global contexts: Each iframe has its own global window, making constructors unique to each frame.
✅ Prototype chain mismatch: Objects created in one frame don’t share the same prototype with objects from another frame.
✅ Avoid instanceof for cross-frame checks: Instead, use Object.getPrototypeOf(), isPrototypeOf(), or Object.prototype.toString.call().
✅ Explicit constructor references: Store shared references in a global scope to ensure consistency.

By understanding and addressing these issues, you can avoid unexpected behavior when working with objects across different execution contexts in JavaScript.