Question 1

var vs let vs const: what is the difference?

Accepted Answer

I default to const, use let when a value changes, and avoid var because of its function scope and hoisting.

Question 2

What is the difference between == and ===?

Accepted Answer

I use === almost everywhere because it avoids hidden type conversion.

Question 3

What is a closure?

Accepted Answer

A closure keeps access to the variables where the function was created, which is how counters and private state work.

Question 4

How does this work, and how are arrow functions different?

Accepted Answer

For regular functions this depends on the call, while arrow functions inherit it, so I use arrows for callbacks.

Question 5

Explain the event loop. In what order do these run?

Accepted Answer

Sync code runs first, then microtasks like promises, then timers: that is the event loop order.

Question 6

Promises vs async/await?

Accepted Answer

async/await is promises with nicer syntax, and I use Promise.all when the calls are independent.

Question 7

What are JavaScript's data types? Why is typeof null 'object'?

Accepted Answer

Primitives are copied by value and objects by reference, and typeof null being 'object' is a historic quirk.

Question 8

map, filter and reduce: what does each do?

Accepted Answer

map transforms, filter selects and reduce combines, and all three return new values without mutating the array.

Question 9

call vs apply vs bind?

Accepted Answer

call and apply invoke now with a chosen this, and bind creates a function with that this for later.

Question 10

What are two classic JavaScript gotchas interviewers love?

Accepted Answer

Default sort is lexicographic and floating-point maths is inexact, so I pass a comparator and compare money in whole units.

	var	let	const
Scope	Function	Block	Block
Before its line	undefined	ReferenceError (temporal dead zone)	ReferenceError
Reassign	Yes	Yes	No (but an object's contents can change)
Use today	Avoid	When it changes	Default choice

JavaScript, in 10 cards

var vs let vs const: what is the difference?