JavaScript Fundamentals: Types, Scope, Closures & the Event Loop

=== (strict equality) compares value and type with no conversion. == converts types first, which causes surprises. Use === by default.

true false true false

53 2 10

object number object function

false true

false

false , 0 , -0 , 0n , "" , null , undefined , NaN . Everything else, including [] , {} and "0" , is truthy.

undefined : a variable was declared but never assigned (or a property or argument is missing). null : an intentional 'no value' that you assign yourself.

var : function-scoped, hoisted and initialized to undefined, can be redeclared. let : block-scoped, can be reassigned. const : block-scoped, cannot be reassigned.

undefined

[1, 2, 3]

3

3 3 3 (on separate lines)

A D C B (on separate lines)

5 0 5

user?.address?.city returns undefined instead of throwing if user or address is null or undefined.

[1, 10, 9]

10

[20, 40]

Mutate: push , pop , shift , unshift , splice , sort , reverse , fill . Return a new array: map , filter , slice , concat , toSorted , toReversed .

1 { b: 2, c: 3 }

2

Arrow functions do not have their own this ; they use the this of the surrounding scope. Regular functions get this from how they are called (e.g. the object before the dot).

true

all : resolves with every result, rejects as soon as one rejects. allSettled : waits for all and reports each outcome. race : settles with the first promise to settle.

Bubbling: an event fires on the target and then on each ancestor up the DOM. Delegation: put one listener on a parent and check event.target, instead of a listener on every child.