Javascript Types: Cheatsheet

A detailed cheatsheet that covers the nitty-gritties of the 6 types of values in javascript.


Javascript has six types of values.

  • Numbers
  • Strings
  • Booleans
  • Objects
  • Functions
  • Undefined values

It’s also useful to know that NaN is a type of number and null is an object.


All numbers in javascript are 64 bit. They use one sign bit and 11 bits to locate the decimal point, allowing numbers upto 252.

1234.5678 // Valid number 12.3e2 // 1230

Nothing really unexpected here except that there’s no int, float, double etc. Just number. Simple.


Strings are what you expect them to be. Any bunch of characters, enclosed in single or double quotes. Characters are escaped or given special meaning using a \ followed by a character. \n, \t, \" all work as expected.


Booleans are - wait for it - either true or false. It’s not so much the data type but the way the operators behave, that makes javascript’s boolean world so awesome and different.


We all know BODMAS and the usual +, -, *, /, % behaviour with numbers.

  • + works on strings as a concatenator
  • - can be used as a unary operator
  • +=, -=, ++ and -- are all part of the language and work as expected

Logical Operations

When you compare two values of the same type, things work as expected. When you perform a logical operation between two values of different types, javascript automatically typecasts then evaluates and throws out a boolean. This scares people. I don’t think it’s all that bad and find it quite intuitive. Let’s try and learn how it works by example.

"5" == 5 // true 0 == "" // true 1 == true // true 0 == false // true null == 0 // false null == false // false null == true // false null == undefined // true NaN == NaN // false

The below conversions take place before the operator is evaluated.

  1. Number vs Boolean

    • Every number except zero is converted to true.
    • Zero translates to a false.
  2. Number vs String

    • The string is converted to a number.
    • "" is converted to a zero.
  3. String and Boolean

    • String is first converted to a number.
    • The number is then converted to boolean.