Step-by-step articles on JavaScript, part of our Frontend Interviews collection.
Showing 10 articles
Short answer: a closure is a function together with the variables from the scope where it was created. The function keeps access to those variables even after the outer function has returned. function makeCounter() { let count = 0; // captured by the inner fun...
Short answer: JavaScript runs on a single thread with a call stack. Async work (timers, network, I/O) is handled by the environment, which queues callbacks. The event loop takes the next callback when the stack is empty, always draining all microtasks before t...
Short answer: === (strict equality) compares value and type with no conversion. == (loose equality) converts the operands to a common type first, which leads to surprising results. 1 === '1' // false - different types 1 == '1' // true - '1' converted to 1 0 ==...
Short answer: var is function-scoped and hoisted with the value undefined . let and const are block-scoped and can't be used before their declaration (the temporal dead zone). const can't be reassigned. Scope if (true) { var a = 1; let b = 2; } console.log(a);...
Short answer: for regular functions, this is decided by how the function is called , not where it's defined. Arrow functions don't have their own this ; they use the this of the surrounding scope. The rules, in order new : new User() - this is the new object....
Short answer: a promise represents a value that will be available later; it's pending, then fulfilled or rejected. async / await is syntax on top of promises: an async function always returns a promise, and await pauses the function until a promise settles. Sa...
Short answer: every JavaScript object has an internal link to another object, its prototype . When you read a property that the object doesn't have, JavaScript looks it up along this prototype chain until it finds it or reaches null . const animal = { speak()...
Short answer: a shallow copy duplicates only the top level; nested objects are still shared. A deep copy duplicates every level, so changing the copy never affects the original. const original = { name: 'Asha', address: { city: 'Pune' } }; const shallow = { .....
Task: write debounce(fn, wait) that returns a function which only calls fn after it stops being called for wait milliseconds. Typical use: search-as-you-type. Solution function debounce(fn, wait) { let timer; return function (...args) { clearTimeout(timer); //...
Short answer: event delegation means attaching one listener to a parent element instead of one per child. It works because most DOM events bubble up from the element where they happened through its ancestors. <ul id="todo-list"> <li data-id="1">Buy milk <butto...
