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Difficulty: 4/10
constructor functions, prototype chain, class syntax
They can be used to create new objects or as Type Transformer.
The Object() constructor turns the input into an object. Its behavior depends on the input's type.
- 1
If the value is null or undefined, it creates and returns an empty object.
- 2
If the value is an object already, it returns the value.
- 3
Otherwise, it returns an object of a type that corresponds to the given value. For example, passing a BigInt primitive returns a BigInt wrapper object
javascript
Ensuring an Input is Object-Safe: If you are writing a function that expects an object but might receive null, calling Object() on the input prevents your code from crashing when you try to access properties.
Coercing Primitives to Objects: Sometimes you need a primitive to behave like an object permanently (not just via temporary autoboxing).
Checking if a value is already an object: Since Object(value) returns the same reference if the input is already an object, you can use it for identity checks.
Scenario Questions
0-2 years experience
- 1We need a simple User object with name and email. Show me how you'd write a constructor function to create User instances and how you'd instantiate one.
- 2If you accidentally call the User constructor without new, what will the result be and how would you guard against it?
- 3How would you add a method to all User instances without redefining it inside the constructor?
2-5 years experience
- 1Our team is refactoring a legacy codebase that uses constructor functions for a Widget. During a bug hunt you notice that Widget.prototype.render is undefined on some instances. Walk me through how you would debug this.
- 2We want to extend a base Shape constructor to create Circle and Rectangle types, each with its own area method while sharing common logic. Explain how you'd set up the inheritance and why you choose that pattern.
- 3A new developer added a line User.call(this, data) inside another constructor but forgot to set the prototype chain. What side effects could this cause and how would you fix it?
5-8 years experience
- 1Our front‑end library creates thousands of component objects per page using a constructor. We're seeing memory pressure and GC churn. How would you evaluate whether using a constructor function, class, or a factory pattern is more performant, and what changes would you recommend?
- 2We need to expose a public API that returns objects created by a constructor but we must prevent consumers from modifying internal prototype methods. Describe a design that isolates the prototype while still using constructors.
- 3During a rollout, a feature toggle disables a new constructor‑based module, but some code still references its prototype methods, causing runtime errors. How would you structure the module to avoid such coupling and make the toggle safe?
8+ years experience
- 1Our product is migrating a massive codebase from ES5 constructor functions to ES6 classes across multiple teams. What migration strategy would you propose to minimize breakage, ensure type safety, and keep performance consistent?
- 2We are designing a shared UI component framework used by several services. Some services need deep inheritance, others prefer composition. How would you decide when to expose a constructor‑based API versus a factory/composition approach, considering long‑term maintainability?
- 3Legacy modules rely on mutating the prototype of built‑in objects via constructors. What architectural guidelines would you set to prevent prototype pollution in a large organization, and how would you enforce them?