What is the Temporal Dead Zone?

The span between entering a scope and a `let` or `const` declaration being initialized, during which access throws

A period where a variable is `undefined`

Does `const` make an object immutable?

No — it prevents reassignment of the binding, not mutation of the value

What is the scope of a `var` declaration inside an `if` block?

Function-scoped, or global if not inside a function

Are function declarations hoisted differently from function expressions?

Declarations are fully hoisted and callable before their definition; expressions follow their variable's hoisting rules

Expressions are hoisted, declarations are not

What does declaring `var x` at the top level of a script do in a browser?

Creates a property on the global object (`window`)

Can you redeclare a `let` variable in the same scope?

No — it throws a `SyntaxError`

What does an arrow function NOT have its own binding for?

`this`, `arguments`, `super`, and `new.target`

Can an arrow function be used with `new`?

No — it throws a `TypeError` because arrows have no `[[Construct]]` method

What does the rest parameter `...args` collect?

The remaining arguments as a real array

All arguments as an array-like object

A function expression invoked immediately at the point of definition

A function that runs on an interval

What does `Function.prototype.call` do?

Calls a function with a given `this` and arguments listed individually

Returns a new bound function without calling it

Calls a function with arguments as an array

What does `bind` return?

A new function permanently bound to the given `this`

The original function's return value

What is a higher-order function?

A function that takes a function as an argument, returns one, or both

A function with more than three parameters

A function defined at the top of a file

What is the difference between a parameter and an argument?

A parameter is the name in the declaration; an argument is the value passed at the call site

Arguments are optional, parameters are not

Parameters are typed, arguments are not

How do you rename a variable while destructuring?

`const { name => label } = obj`

`const { name as label } = obj`

Is the spread operator a deep or shallow copy?

Shallow — nested objects are shared by reference

What does the rest element in object destructuring collect?

The remaining own enumerable properties not already destructured

Can you destructure a function parameter directly?

Yes, including with defaults: `function f({ a = 1 } = {})`

Can you spread a string?

Yes — it produces an array of characters, since strings are iterable

What is the prototype chain?

The chain of objects JavaScript walks when looking up a property that an object does not own

The inheritance order of classes only

What is the relationship between `class` and prototypes?

`class` is syntax sugar over prototype-based inheritance

Classes are compiled to functions only in strict mode

Classes replaced prototypes entirely

Classes use a separate inheritance system

What does `#count` in a class body declare?

A truly private field, inaccessible outside the class

What does the `static` keyword do on a class member?

Attaches it to the class itself rather than to instances

What does `Object.freeze` prevent?

Adding, removing, and reassigning own properties — but only one level deep

All changes, including nested objects

What does `Object.entries(obj)` return?

An array of `[key, value]` pairs

What is the difference between `Object.keys` and `for...in`?

`Object.keys` returns only own enumerable properties; `for...in` also walks inherited ones

What are getters and setters in a class or object literal?

Accessor properties that run a function on property read or write

Methods that must be called explicitly

What does `structuredClone` do that spread does not?

Creates a deep copy, including nested objects, Maps, Sets, and Dates

What is `Object.create(null)` useful for?

Creating an object with no prototype, safe for use as a plain dictionary

What is `this` inside an arrow function defined in an object literal method position?

Whatever `this` is in the enclosing lexical scope, often the module or global scope

Why does this log `undefined`? ```js const obj = { name: 'A', get() { return this.name; } }; const fn = obj.get; fn(); ```

Extracting the method loses the `this` binding, so `this` is not `obj`

What is `this` inside a class method called normally?

The instance the method was called on

How do class fields with arrow functions change binding?

The arrow captures `this` from the instance at construction, so the method stays bound when extracted

What does `globalThis` provide?

A standard way to access the global object across browsers, Node, and workers

The `this` of the calling function

A reference to the current module

What is `this` inside a `setTimeout` callback written as a regular function?

Not the enclosing object — it is the timer context, `undefined` in strict mode

All practice tests

JavaScript Practice Test

120 original practice questions across 13 topics, every answer explained. Study mode shows the reasoning as you go; test mode holds it until the end.

Questions: 120
Topics: 13
Intermediate: 120

Read what these tests actually cover

120 questions

01
What is the Temporal Dead Zone?
02
What does console.log(typeof x) produce if x has never been declared?
03
Does const make an object immutable?
04
What is the scope of a var declaration inside an if block?
05
What does this log?
console.log(a); var a = 5;
06
Are function declarations hoisted differently from function expressions?
07
What does declaring var x at the top level of a script do in a browser?
08
What happens when you assign to an undeclared variable in strict mode?

What is the output?

let x = 1;
{
  let x = 2;
  console.log(x);
}
console.log(x);

10
Can you redeclare a let variable in the same scope?
11
What does an arrow function NOT have its own binding for?
12
Can an arrow function be used with new?
13
What does a default parameter value trigger on?
14
What does the rest parameter ...args collect?
15
What is an IIFE?
16
What does Function.prototype.call do?
17
What does bind return?
18
What is a higher-order function?
19
What does this return?
function f() { return { value: 1 }; }
20
What is the difference between a parameter and an argument?
21
What does const { a = 5 } = { a: null } assign to a?
22
How do you rename a variable while destructuring?
23
Is the spread operator a deep or shallow copy?
24
What does const [a, , c] = [1, 2, 3] assign?
25
How do you swap two variables using destructuring?
26
What does the rest element in object destructuring collect?
27
Can you destructure a function parameter directly?
28
What does Math.max(...[1, 5, 3]) return?
29
What happens with duplicate keys when spreading two objects?
30
Can you spread a string?
31
What is the prototype chain?
32
What is the relationship between class and prototypes?
33
What does #count in a class body declare?
34
What does the static keyword do on a class member?
35
What must you call before using this in a derived class constructor?
36
What does Object.freeze prevent?
37
What does Object.entries(obj) return?
38
What is the difference between Object.keys and for...in?
39
What are getters and setters in a class or object literal?
40
What does optional chaining ?. return when the left side is null or undefined?
41
What does structuredClone do that spread does not?
42
What is Object.create(null) useful for?
43
What determines the value of this in a regular function call?
44
What is this inside an arrow function defined in an object literal method position?
45
Why does this log undefined?
const obj = { name: 'A', get() { return this.name; } }; const fn = obj.get; fn();
46
What is this inside a class method called normally?
47
How do class fields with arrow functions change binding?
48
What is this at the top level of an ES module?
49
What does globalThis provide?
50
What is this inside a setTimeout callback written as a regular function?
51
What is a closure?

What does this log?

for (var i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 0);
}

53
Why does let fix the loop closure problem?
54
What is the module pattern?
55
What does a function returning a function that remembers a value demonstrate?
const counter = () => { let n = 0; return () => ++n; };
56
What is currying?
57
What is memoization?
58
Can closures cause memory leaks?
59
Does Array.prototype.sort mutate the original array?
60
What does [10, 9, 1].sort() produce without a comparator?
61
What is the difference between slice and splice?
62
Can you break out of a forEach loop?
63
What does reduce do when called on an empty array with no initial value?
64
What does map return when the callback has no explicit return?
65
What does Array.from do that spread does not?
66
What does flat(Infinity) do?
67
What is the difference between find and filter?
68
What does at(-1) return?
69
What do the ES2023 methods toSorted, toReversed, toSpliced, and with have in common?
70
What does Object.groupBy (ES2024) return?
71
What are the three states of a Promise?
72
What does an async function always return?
73
What is the difference between Promise.all and Promise.allSettled?
74
What does Promise.race resolve with?
75
What does Promise.any reject with if every input rejects?
76
What is wrong with this?
const results = []; for (const url of urls) { results.push(await fetch(url)); }
77
What happens to an unhandled promise rejection in Node.js?
78
Can try/catch catch an error from a promise you did not await?
79
What does .finally() receive as an argument?
80
What does chaining .then() return?
81
What does await do to a non-promise value?
82
How do you add a timeout to a fetch request?
83
What is a promise chain anti-pattern often called the "explicit construction antipattern"?
84
What does top-level await require?

What is the output order?

console.log('A');
setTimeout(() => console.log('B'), 0);
Promise.resolve().then(() => console.log('C'));
console.log('D');

86
Which queue has higher priority: microtasks or macrotasks?
87
Does setTimeout(fn, 0) run immediately?
88
Is JavaScript single-threaded?
89
What happens when a long synchronous computation runs in the browser?
90
What is queueMicrotask used for?
91
What does process.nextTick do in Node relative to promises?

What is the output?

async function f() {
  console.log(1);
  await null;
  console.log(2);
}
f();
console.log(3);

93
What is a key difference between ES modules and CommonJS?
94
What does "live bindings" mean for ES module exports?
95
How many times does a module's top-level code execute across multiple imports?
96
What is the difference between a default and a named export?
97
What does import() (the function form) return?
98
Are ES modules in strict mode?
99
How do you designate a Node package as ESM?
100
What is tree shaking?
101
What makes an object iterable?
102
What does a generator function's yield do?
103
What does calling a generator function return?
104
What is the shape of the object returned by next()?
105
What is Symbol used for?
106
Are symbol-keyed properties included in Object.keys or JSON.stringify?
107
What is the difference between Map and a plain object?
108
What distinguishes WeakMap from Map?
109
What does typeof null return?
110
What is the difference between == and ===?
111
What does NaN === NaN evaluate to?
112
How does Object.is differ from ===?
113
Which values are falsy in JavaScript?
114
What is the difference between ?? and ||?
115
What do the logical assignment operators ||=, &&=, and ??= do?
116
What is optional catch binding?
117
What is the cause option on the Error constructor?
118
What is BigInt for?
119
What does a finally block do if the try block returns?
120
Why should you avoid JSON.parse(JSON.stringify(obj)) for deep cloning?

All practice tests

JavaScript Practice Test

120 original practice questions across 13 topics, every answer explained. Study mode shows the reasoning as you go; test mode holds it until the end.

Questions: 120
Topics: 13
Intermediate: 120

Read what these tests actually cover

120 questions

01
What is the Temporal Dead Zone?
02
What does console.log(typeof x) produce if x has never been declared?
03
Does const make an object immutable?
04
What is the scope of a var declaration inside an if block?
05
What does this log?
console.log(a); var a = 5;
06
Are function declarations hoisted differently from function expressions?
07
What does declaring var x at the top level of a script do in a browser?
08
What happens when you assign to an undeclared variable in strict mode?

What is the output?

let x = 1;
{
  let x = 2;
  console.log(x);
}
console.log(x);

10
Can you redeclare a let variable in the same scope?
11
What does an arrow function NOT have its own binding for?
12
Can an arrow function be used with new?
13
What does a default parameter value trigger on?
14
What does the rest parameter ...args collect?
15
What is an IIFE?
16
What does Function.prototype.call do?
17
What does bind return?
18
What is a higher-order function?
19
What does this return?
function f() { return { value: 1 }; }
20
What is the difference between a parameter and an argument?
21
What does const { a = 5 } = { a: null } assign to a?
22
How do you rename a variable while destructuring?
23
Is the spread operator a deep or shallow copy?
24
What does const [a, , c] = [1, 2, 3] assign?
25
How do you swap two variables using destructuring?
26
What does the rest element in object destructuring collect?
27
Can you destructure a function parameter directly?
28
What does Math.max(...[1, 5, 3]) return?
29
What happens with duplicate keys when spreading two objects?
30
Can you spread a string?
31
What is the prototype chain?
32
What is the relationship between class and prototypes?
33
What does #count in a class body declare?
34
What does the static keyword do on a class member?
35
What must you call before using this in a derived class constructor?
36
What does Object.freeze prevent?
37
What does Object.entries(obj) return?
38
What is the difference between Object.keys and for...in?
39
What are getters and setters in a class or object literal?
40
What does optional chaining ?. return when the left side is null or undefined?
41
What does structuredClone do that spread does not?
42
What is Object.create(null) useful for?
43
What determines the value of this in a regular function call?
44
What is this inside an arrow function defined in an object literal method position?
45
Why does this log undefined?
const obj = { name: 'A', get() { return this.name; } }; const fn = obj.get; fn();
46
What is this inside a class method called normally?
47
How do class fields with arrow functions change binding?
48
What is this at the top level of an ES module?
49
What does globalThis provide?
50
What is this inside a setTimeout callback written as a regular function?
51
What is a closure?

What does this log?

for (var i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 0);
}

53
Why does let fix the loop closure problem?
54
What is the module pattern?
55
What does a function returning a function that remembers a value demonstrate?
const counter = () => { let n = 0; return () => ++n; };
56
What is currying?
57
What is memoization?
58
Can closures cause memory leaks?
59
Does Array.prototype.sort mutate the original array?
60
What does [10, 9, 1].sort() produce without a comparator?
61
What is the difference between slice and splice?
62
Can you break out of a forEach loop?
63
What does reduce do when called on an empty array with no initial value?
64
What does map return when the callback has no explicit return?
65
What does Array.from do that spread does not?
66
What does flat(Infinity) do?
67
What is the difference between find and filter?
68
What does at(-1) return?
69
What do the ES2023 methods toSorted, toReversed, toSpliced, and with have in common?
70
What does Object.groupBy (ES2024) return?
71
What are the three states of a Promise?
72
What does an async function always return?
73
What is the difference between Promise.all and Promise.allSettled?
74
What does Promise.race resolve with?
75
What does Promise.any reject with if every input rejects?
76
What is wrong with this?
const results = []; for (const url of urls) { results.push(await fetch(url)); }
77
What happens to an unhandled promise rejection in Node.js?
78
Can try/catch catch an error from a promise you did not await?
79
What does .finally() receive as an argument?
80
What does chaining .then() return?
81
What does await do to a non-promise value?
82
How do you add a timeout to a fetch request?
83
What is a promise chain anti-pattern often called the "explicit construction antipattern"?
84
What does top-level await require?

What is the output order?

console.log('A');
setTimeout(() => console.log('B'), 0);
Promise.resolve().then(() => console.log('C'));
console.log('D');

86
Which queue has higher priority: microtasks or macrotasks?
87
Does setTimeout(fn, 0) run immediately?
88
Is JavaScript single-threaded?
89
What happens when a long synchronous computation runs in the browser?
90
What is queueMicrotask used for?
91
What does process.nextTick do in Node relative to promises?

What is the output?

async function f() {
  console.log(1);
  await null;
  console.log(2);
}
f();
console.log(3);

93
What is a key difference between ES modules and CommonJS?
94
What does "live bindings" mean for ES module exports?
95
How many times does a module's top-level code execute across multiple imports?
96
What is the difference between a default and a named export?
97
What does import() (the function form) return?
98
Are ES modules in strict mode?
99
How do you designate a Node package as ESM?
100
What is tree shaking?
101
What makes an object iterable?
102
What does a generator function's yield do?
103
What does calling a generator function return?
104
What is the shape of the object returned by next()?
105
What is Symbol used for?
106
Are symbol-keyed properties included in Object.keys or JSON.stringify?
107
What is the difference between Map and a plain object?
108
What distinguishes WeakMap from Map?
109
What does typeof null return?
110
What is the difference between == and ===?
111
What does NaN === NaN evaluate to?
112
How does Object.is differ from ===?
113
Which values are falsy in JavaScript?
114
What is the difference between ?? and ||?
115
What do the logical assignment operators ||=, &&=, and ??= do?
116
What is optional catch binding?
117
What is the cause option on the Error constructor?
118
What is BigInt for?
119
What does a finally block do if the try block returns?
120
Why should you avoid JSON.parse(JSON.stringify(obj)) for deep cloning?