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A prototype in JavaScript is an object that acts as a blueprint for other objects. Objects can inherit properties and methods from their prototypes, creating a prototype chain. An object can be used as a prototype to create a new object.
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Every object has a property called [[Prototype]]. This property references another object, known as the object's prototype. When an object is created, its [[Prototype]] property is set to the prototype of its constructor function.
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The prototype of an object points to another object or to null —
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Properties of the object's [[Prototype]] can also be accessed on the object itself.

Difficulty: 5/10

Topics: prototype chain, inheritance, object creation

0-2 years experience

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We need to add a method reset() to all objects created by a constructor User. How would you use the prototype to achieve this without modifying each instance?
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If you create an object with Object.create(null), what will happen when you try to access a property that isn’t defined on it? Explain why.

2-5 years experience

1
You notice that a new property added to Array.prototype is causing unexpected behavior in a third‑party library. How would you debug and resolve the issue?
2
During a feature rollout, you need to share a utility function across many objects but want to avoid memory overhead. Would you put it on the prototype or on each instance? Explain the trade‑offs.

5-8 years experience

1
Our front‑end codebase mixes ES6 classes and traditional prototype inheritance. How would you refactor a module that heavily relies on prototype manipulation to improve maintainability and performance?
2
We have a high‑traffic component that creates thousands of objects per second. Discuss the impact of prototype chain depth on V8 performance and how you would mitigate any issues.

8+ years experience

1
The company is migrating a legacy codebase that uses manual prototype chaining to a modern TypeScript class hierarchy. What strategy would you propose to ensure a smooth transition while preserving runtime behavior?
2
Across multiple teams, some services extend core domain objects via prototype augmentation, leading to hidden coupling. How would you establish guidelines or architectural patterns to manage prototype usage at scale?

❓Can you walk me through how you'd test that a prototype change doesn't break existing functionality?
❓What are the risks of augmenting built‑in prototypes in a shared library?
❓How do property descriptor attributes like `enumerable` and `configurable` interact with prototype inheritance?