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Javascript Datatypes


typeof is the 'official' function that one uses to get the type in JavaScript, however in certain cases it might yield some unexpected results ...

1. Strings

typeof "String" or
typeof Date(2011,01,01)

2. Numbers

typeof 42

3. Bool

typeof true (valid values true and false)

4. Object

typeof {} or
typeof [] or
typeof null or
typeof /aaa/ or
typeof Error()

5. Function

typeof function(){}

6. Undefined

var var1; typeof var1

Finding an object's class

To find whether an object was constructed by a certain constructor or one inheriting from it, you can use the instanceof command:

function sum(...arguments) {
 if (arguments.length === 1) {
 const [firstArg] = arguments
 if (firstArg instanceof Array) { //firstArg is something like [1, 2, 3]
 return sum(...firstArg) //calls sum(1, 2, 3)
 return arguments.reduce((a, b) => a + b)
console.log(sum(1, 2, 3)) //6
console.log(sum([1, 2, 3])) //6
console.log(sum(4)) //4

Note that primitive values are not considered instances of any class:

console.log(2 instanceof Number) //false
console.log('abc' instanceof String) //false
console.log(true instanceof Boolean) //false
console.log(Symbol() instanceof Symbol) //false

Every value in JavaScript besides null and undefined also has a constructor property storing the function that was used to construct it. This even works with primitives.

console.log([] instanceof Object, [] instanceof Array) //true true
console.log([].constructor === Object, [].constructor === Array) //false true
function isNumber(value) {
 //null.constructor and undefined.constructor throw an error when accessed
 if (value === null || value === undefined) return false
 return value.constructor === Number
console.log(isNumber(null), isNumber(undefined)) //false false
console.log(isNumber('abc'), isNumber([]), isNumber(() => 1)) //false false false
console.log(isNumber(0), isNumber(Number('10.1')), isNumber(NaN)) //true true true


In this page (written and validated by ) you learned about Javascript Datatypes . What's Next? If you are interested in completing Javascript tutorial, your next topic will be learning about: Javascript object type by constructor name.

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