JavaScript language fundamentals

Frontend loops usually combine a JavaScript fundamentals screen, a UI coding round, a frontend system design round and behavioral interviews. This guide covers the frontend-specific parts; the JavaScript and TypeScript options in the problem workspace cover algorithm rounds. Reviewed October 11, 2026.

Scope and closures

let and const are block scoped; var is function scoped and hoisted. A closure is a function together with the variables it captured from its defining scope.

function counter() {
  let count = 0;
  return { increment: () => ++count, value: () => count };
}
const c = counter();
c.increment(); c.increment();
c.value(); // 2

The classic loop bug: with var, every callback shares one i; with let, each iteration gets its own binding.

for (var i = 0; i < 3; i++) setTimeout(() => console.log(i)); // 3 3 3
for (let j = 0; j < 3; j++) setTimeout(() => console.log(j)); // 0 1 2

this

this is decided by how a function is called: as a method (obj.f()), plainly (undefined in strict mode), with new, or explicitly with call, apply or bind. Arrow functions do not have their own this; they use the enclosing one, which is why they are convenient for callbacks inside methods.

Prototypes and classes

Objects delegate property lookups along their prototype chain. class syntax is a clearer way to set up the same prototype relationships.

Equality and types

Use ===; == applies coercion rules that surprise people (0 == "" is true). typeof null is "object", NaN !== NaN, and Object.is distinguishes +0 and -0.

Common implementation questions

Implement debounce, throttle, Promise.all, a deep clone, Function.prototype.bind, an event emitter, and curry. Each tests closures and careful handling of this and arguments.

function debounce(fn, wait) {
  let timer;
  return function (...args) {
    clearTimeout(timer);
    timer = setTimeout(() => fn.apply(this, args), wait);
  };
}