Data Types

Javascript is a loosly typed language and, like many languages, contains primitive and non-primitive types.

Primitives

In JavaScript primitives are defined as "data that is not on object and has no methods". The values themselves cannot have methods and are therefore immutable. They can only be altered by re-assignment.

• String
  • A collection of characters contained within quotation marks (single or double).

  const name = 'John';

• Number
  • Can be an integer or float

  const age = 25;

• Boolean
  • True or false

  let isOpen = true;

• Undefined
  • The default value for variables before they’re assigned a value

  let x; // x is of type undefined at this point.
  
  x = 5; // x no longer has a type of undefined, instead it's now a number.

• Null

  let x = null;

• Symbol (ES6)
  • Symbol is a built-in object whose constructor returns a symbol primitive — also called a Symbol value or just a Symbol that’s guaranteed to be unique
  • Symbols are often used to add unique property keys to an object that won’t collide with keys any other code might add to the object

  let sym1 = Symbol()
  let sym2 = Symbol('foo')

• BigInt (ES2020)
  • Used to represent whole numbers with a value larger than 2^53 - 1 (~ 9 quadrillion)
  • Can be created by either appending a "n" to the end of the integer literal, or by using the BigInt() constructor

  const previouslyMaxInteger = 9007199254740991n
  const anotherHuge = BigInt(9007199254740991)

  • BigInt values cannot be used with the built-in Math object
  • They also cannot be mixed with a Number value in operations.

  let num = 123;
  let bigInt = BigInt(9007199254740992);
  // This is not allowed, and will result in an error.
  let newValue = num + bigInt;
  // You must convert all values into BigInt types.
  let newValue = BigInt(num) + bigInt;

Non-Primitives

• Array
  • A collection data that is numerically indexed
  • They can contain items of any data type

  let fruits = ['apple', 'orange'];
  let mixedArr = ['red', 7, false];

  • When addressing arrays you must do so using the numerical index

  let fruits = ['apple', 'orange'];
  // This will print 'apple'.
  console.log(fruits[0]);
  // This will print 'orange'.
  console.log(fruits[1]);

• Object
  • A collection of keys-value pairs referred to as properies
  • The data is stored in an unordered fashion, unlike arrays
  • The keys must always be a String
  • The values can be of any data type including other objects or functions

  let animals = {
    'mammals': ['elephant', 'bear', 'whale'],
    'reptiles': ['alligator', 'lizard', 'turtle']
    'birds': ['ostrich', 'flamingo', 'pigeon']
  };

Arrays and Objects will be discussed in more detail in another lesson (link tbd).

References:

1. Javascript data types