easy== vs equals
What is the result?
let arr = ["a", "b", "c"];
arr.reverse();
console.log(arr);JavaScript Interview Questions (580 to practise)
JavaScript runs in every browser and, through Node, on most back ends too. It is the language with the widest reach and the most edge cases.
Interviewers lean on coercion rules, the prototype chain, closures over loop variables, and the exact order the microtask queue drains. Most JavaScript screens are really output-prediction screens.
- Total questions
- 580
- Easy
- 110
- Medium
- 221
- Hard
- 249
Where it shows up: Frontend roles at almost every company, plus full-stack and Node back-end roles. It is the most common first round in web hiring.
Sample JavaScript questions
Twenty real JavaScript questions from the library, code snippet included. Nothing here is paraphrased for search engines. It is the same text a signed-in user sees.
easyarray map filter reduce
What will this print?
let arr = ["a", "b"];
arr.push("c");
console.log(arr);easyprimitives and typeof
What does the following code return?
typeof null === "object"easyarray map filter reduce
What is the output?
let arr = [[1, 2], [3, 4, 5]];
console.log(arr.flat());easythis in callbacks vs methods
What will this code output?
const obj = {
name: "Hello",
sayHello: function() {
console.log(this.name);
}
};
obj.sayHello();easytemplate literals
What is the output?
let str = "Hello World";
console.log(str.toUpperCase());easyarray map filter reduce
What is the output?
let numbers = [1, 2, 3, 4];
let result = numbers.find(n => n > 1);
console.log(result);easyarray map filter reduce
What does the following code output?
let fruits = ["apple", "banana", "orange"];
console.log(fruits.indexOf("banana"));easy== vs equals
What is the result?
let add = (x, y) => x + y;
console.log(add(5, 10));easyarray map filter reduce
What is the output?
let numbers = [1, 2, 3, 4];
let sum = numbers.reduce((acc, val) => acc + val);
console.log(sum);easyarray map filter reduce
What will this code log?
let arr = [[1, 2], [3, 4, 5]];
console.log(arr.flatMap(x => x));easyarray map filter reduce
What is the output?
let arr = [1, 2, 3];
let result = arr.map(x => x + 1);
console.log(result);easy== vs equals
What is the result?
undefined * 2easytemplate literals
What will this print?
let str = "world";
console.log(str.endsWith("ld"));easyarray map filter reduce
What is the output?
let arr = [1, 2, 3];
let reversed = arr.slice().reverse();
console.log(reversed);easyprimitives and typeof
What will be the output?
typeof function() {}medium== vs equals
What is the result?
parseInt("5px")easyarray map filter reduce
What will this print?
let arr = [1, 2, 3, 4];
console.log(arr.some(x => x > 2));easy== vs equals
What is the result?
let arr1 = ["a", "b"];
let arr2 = ["c", "d"];
let result = arr1.concat(arr2);
console.log(result);mediumlet const var and scope
What does the following code log?
for (var i = 0; i < 3; i++) {
setTimeout(() => console.log(i), 100);
}Sign in to answer
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JavaScript topics covered
42 distinct JavaScript topics are tagged across the library. These are the real topic labels stored on the questions, not a hand-written list.
- array map filter reduce (51)
- primitives and typeof (46)
- ToPrimitive coercion algorithm (28)
- let const var and scope (25)
- template literals (21)
- Promise.all race allSettled (20)
- object destructuring (18)
- prototype chain internals (18)
- this in callbacks vs methods (17)
- structuredClone (15)
- this-binding spec edge cases (15)
- property descriptors (15)
- Map Set WeakMap (14)
- Symbol registry (14)
- immutability patterns (13)
- getters and setters (13)
- SharedArrayBuffer and Atomics (13)
- microtask vs macrotask ordering (12)
- generators and yield (12)
- typed arrays and ArrayBuffer (12)
- Proxy and Reflect (12)
- generator return throw protocol (12)
- closures as a pattern (11)
- ES module resolution algorithm (11)
- custom iterables Symbol.iterator (11)
- hidden classes and inline caches (11)
- Error subclassing (10)
- == vs equals (9)
- closures and memory retention (9)
- recursion and stack limits (9)
- top-level await ordering (9)
- practical event-loop model (8)
- WeakRef and FinalizationRegistry (8)
- garbage collection and leaks (8)
- memoization (7)
- bitwise operators (6)
- debounce and throttle (6)
- tagged templates (5)
- currying (5)
- requestAnimationFrame timing (4)
- tail-call behavior (4)
- Object.create null (3)
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