20 Essential JavaScript Interview Questions for Top Tech Companies

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JavaScript is a powerful language, but mastering its internals is key to cracking interviews at top product-based companies (PBCs). Below are 20 critical questions that test your deep understanding of JavaScript, along with practical insights.

  1. What is Just-In-Time (JIT) Compilation and How Does It Optimize Performance?
    JIT compilation converts JavaScript code into machine code at runtime, improving execution speed. V8 uses baseline (quick, less optimized) and optimizing (slower, highly efficient) compilers.

You Should Know:

 Check V8 optimization status in Node.js 
node --trace-opt script.js 
node --trace-deopt script.js
  1. How Do Hidden Classes and Inline Caching Impact App Speed?
    Hidden classes optimize property access. Changing object shapes (adding/deleting properties) degrades performance.

You Should Know:

// Bad: Dynamic property addition 
let obj = {}; 
obj.a = 1; 
obj.b = 2;

// Good: Fixed structure 
let obj = { a: 1, b: 2 };
  1. What is the Event Loop and How Do Microtasks vs Macrotasks Execute?

Microtasks (Promises, `queueMicrotask`) run before macrotasks (`setTimeout`, `setInterval`).

You Should Know:

console.log("Start"); 
setTimeout(() => console.log("Timeout"), 0); 
Promise.resolve().then(() => console.log("Promise")); 
console.log("End"); 
// Output: Start, End, Promise, Timeout
  1. How Do Closures Work and Can They Leak Memory?
    Closures retain outer function scope. Improper use can cause memory leaks.

You Should Know:

function outer() { 
let data = "Sensitive"; 
return function inner() { 
console.log(data); // data retained 
}; 
} 
const leak = outer();
  1. What is the Temporal Dead Zone (TDZ)?
    `let` and `const` are hoisted but inaccessible before declaration.

You Should Know:

console.log(a); // ReferenceError (TDZ) 
let a = 10;
  1. How Does JavaScript Manage Memory in Heap vs Stack?
    Primitives (numbers, strings) go to the stack. Objects, functions go to the heap.

You Should Know:

 Monitor memory in Node.js 
node --inspect script.js
  1. How Do WeakMap and WeakSet Help Avoid Memory Leaks?

They hold weak references, allowing garbage collection.

You Should Know:

let obj = {}; 
const weakMap = new WeakMap(); 
weakMap.set(obj, "data"); 
obj = null; // Entry removed from WeakMap
  1. How Does the `this` Keyword Behave in Arrow vs Regular Functions?

Arrow functions inherit `this` from the parent scope.

You Should Know:

const obj = { 
name: "JS", 
regularFunc: function() { console.log(this.name); }, 
arrowFunc: () => console.log(this.name) 
}; 
obj.regularFunc(); // "JS" 
obj.arrowFunc(); // undefined (or global <code>this</code>)
  1. How Would You Implement Debounce or Throttle from Scratch?

Debounce delays execution until after a pause.

You Should Know:

function debounce(func, delay) { 
let timeout; 
return (...args) => { 
clearTimeout(timeout); 
timeout = setTimeout(() => func(...args), delay); 
}; 
}
  1. What Are Typed Arrays and How Do They Enhance Performance?

Typed arrays (`Int32Array`, `Float64Array`) handle binary data efficiently.

You Should Know:

const buffer = new ArrayBuffer(16); 
const int32View = new Int32Array(buffer);

What Undercode Say

Mastering JavaScript internals is crucial for high-performance applications and interviews. Use tools like:
– Chrome DevTools (Performance tab)
– Node.js Inspector (--inspect)
– Memory Profiling (heap snapshots)

Expected Output:

A deep understanding of JavaScript’s execution model, memory management, and optimization techniques will set you apart in technical interviews.

For structured learning, check out:

Prediction

As JavaScript evolves, expect more optimizations around WebAssembly integration, parallel processing, and memory-efficient data structures in future ECMAScript versions.

References:

Reported By: Sakshi Gawande – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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