Javascript Datatypes
Typeof
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 423. 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 var1Finding 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)) //4Note 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) //falseEvery 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