JS Types

A type is a built-in JavaScript object that handles a specific type of data.

These types include:

  • String - a group of characters.
  • Number - any combination of numbers.
  • Boolean - returns true or false.
  • null - intentional absence of an object value.
  • undefined - a variable declared but not given a value (automatic).
  • Symbol - this is a unique type generated by the Symbol() function.
const aString = "John Smith";
const aNumber = 2000;
const aBoolean = true;
const aNullType = null;
const anUndefinedType = undefined;
Let undefinedIsAutomatic; // All variables start out as "undefined"

To determine an object's type, typeOf  can be used:

typeOf aString: string
typeOf aNumber: number
typeOf aBoolean: boolen
typeOf aNullType: null
typeOf anUndefinedType: undefined
typeOf undefinedIsAutomatic: undefined


In JavaScript, the string data type is a data sequence consisting of numbers, letters and/or special characters. Below are examples of a string and the various methods available for retrieving info or altering.

const aString = "Hello World!";
aString.length // 12
aString.indexOf('o') //
aString.lastIndexOf('o') // 7
aString.toLowerCase() // "hello world!"
aString.toUpperCase() // "HELLO WORLD!"
aString.bold() // "<b>Hello World!</b>"
aString.replace('H', 'Z') // "Zello World!"
aString.replace('World', 'JavaScript') // "Hello JavaScript!"


Common in computer science, a Boolean data type provides a "on/off" sort of data value, often if the form of true or false. In JavaScript, a common use/case would be to set up a conditional(if this happens, then do this) to decide if a group of code should run, and then use a Boolean return of "true" to run or "false" not to run the code. Below are the results of creating a boolean with different types of data. Essentially, 0, an empty string, false Nan, null and undefined register as false and any other value is true, including a string with the word "false".

var b1 = new Boolean(0); // false
var b2 = new Boolean(1); // true
var b3 = new Boolean(false); // false
var b4 = new Boolean(""); // false
var b5 = new Boolean("false"); // true
var b6 = new Boolean(NaN); // false
var b7 = new Boolean(null); // false
var b8 = new Boolean("Hello"); // true
var b9 = new Boolean(-0.1); // true


null is the absence of any value. This missing value would usually be intentional. If null is found when performing a boolean operation, the return would be "false".

const aNullType = null;  //  null


undefined is just what the name indicates: no value has been defined. this is the automatic state of a variable until it is assigned any value. If undefined is found when performing a boolean operation, the return would be "false".

const anUndefinedType = undefined;  // undefined
let undefinedIsAutomatic;  // undefined


Symbol() is used to create a completely unique type. This can be used to prevent conflicts, especially in an object with a large number of properties. In these cases Symbol() can be used to create custom identifiers ensuring they will be completely unique.

//	without Symbol()
let sym1 = "foo";
let sym2 = "foo";
console.log(sym1 === sym2);RESULT: true;

//	WIth Symbol()
let sym1 = Symbol('foo');
let sym2 = Symbol('foo');
console.log(sym1 === sym2); //	RESULT: false;

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