Some JavaScript dot points of this and that (details refer to MDN).


There are 7 data types as defined in ECMAScript standard:

  • 6 primitives (primitive values are immutable):

    • Boolean
    • Null
    • Undefined
    • Number
    • String
    • Symbol (new in ECMAScript 6)
  • Object (In JavaScript, objects can be seen as a collection of properties)

Primitive Types Primitive Wrapper Object typeof Value
Boolean Boolean typeof boolean = “undefined” true, false
Number Number typeof number = “undefined” -(2^53 -1) to 2^53 -1 and 3 symbolic values: +Infinity, -Infinity, and NaN
String String typeof string = “undefined” a set of “elements” of 16-bit unsigned integer values
Symbol Symbol typeof symbol = “undefined” a unique and immutable; may be used as the key of an Object property
Null   typeof null = “object” null
Undefined   typeof undefined = “undefined” undefined

Map vs. Set

Map objects hold key-value pairs. Key equality is based on the “SameValueZero” algorithm: NaN is considered the same as NaN (even though NaN !== NaN) and all other values are considered equal according to the semantics of the === operator.

Set objects store unique values of any type. NaN and undefined can also be stored in a Set. NaN is considered the same as NaN (even though NaN !== NaN).

For both map and set, -0 and +0 are considered equal in the current ECMAScript specification but were not so in earlier drafts.

Map vs. Object

  • The keys of an Object are Strings and Symbols, whereas they can be any value for a Map, including functions, objects, and any primitive.
  • You can get the size of a Map easily with the size property, while the number of properties in an Object must be determined manually.
  • A Map is an iterable and can thus be directly iterated, whereas iterating over an Object requires obtaining its keys in some fashion and iterating over them.
  • An Object has a prototype, so there are default keys in the map that could collide with your keys if you’re not careful. As of ES5 this can be bypassed by using map = Object.create(null), but this is seldom done.
  • A Map may perform better in scenarios involving frequent addition and removal of key pairs.