Interview Q&A / JavaScript

1. How does the this keyword work in JavaScript?

   this is decided by how a function is called, not where it is written. A method call obj.fn() sets this to obj; new sets it to the new object; call, apply and bind set it explicitly; a plain call gives undefined in strict mode. Arrow functions have no own this and use the surrounding one.

   IntermediateIn-depth3 min read
2. What are prototypes in JavaScript, and how does prototypal inheritance work?

   Every object has a hidden link to another object called its prototype. When you read a property that the object does not have, JavaScript looks it up along this prototype chain. Classes are syntax built on top of this: methods live on the prototype and are shared by all instances.

   AdvancedIn-depth3 min read
3. How do you check whether a property exists on an object?

   Use Object.hasOwn(obj, key) for own properties and the in operator to include inherited ones. Comparing to undefined is unreliable, because a property can exist and hold undefined.

   BeginnerQuick answer1 min read
4. Is JavaScript pass by value or pass by reference?

   JavaScript is always pass by value. For objects, the value passed is a reference, so a function can change the object's contents but cannot make the caller's variable point to a different object.

   BeginnerQuick answer1 min read
5. What do Object.keys, Object.values, Object.entries and Object.fromEntries do?

   keys gives an array of own enumerable property names, values gives their values, and entries gives [key, value] pairs. fromEntries does the reverse and builds an object from pairs, which makes transforming objects easy.

   BeginnerQuick answer1 min read
6. What is the difference between a shallow copy and a deep copy of an object?

   A shallow copy duplicates only the top level, so nested objects are still shared with the original. A deep copy duplicates every level, so the two are fully independent. Spread and Object.assign are shallow; structuredClone is deep.

   BeginnerQuick answer1 min read
7. What is the difference between Object.freeze, Object.seal and Object.preventExtensions?

   preventExtensions blocks adding properties. seal also blocks deleting and reconfiguring, but allows changing values. freeze blocks everything, so values are read-only too. All three are shallow.

   IntermediateQuick answer1 min read
8. Write a function that deep clones an object, including nested arrays and circular references.

   Return primitives as they are. For objects and arrays, create an empty copy, record it in a WeakMap so circular references reuse it, then recursively clone each property. In modern runtimes, structuredClone does this for you.

   IntermediateQuick answerCoding1 min read