Question

I need to check the type of a variable in JavaScript. I know 3 ways to do it:

1. instanceof operator: if(a instanceof Function)

2. typeof operator: if(typeof a=="function"

3. toString method (jQuery uses this): Object.prototype.toString.call(a) == "[object Function]"

Which is the most accurate way to do type checking beetween these solutions? And why? Please don’t tell me that the last solution is better only because jQuery uses that.

Answer 1

[Edit 2023/05/28] See this github repository for a more comprehensive type checker.

How about my home brew function to determine variable ‘type’? It can also determine the type of custom Objects:

function whatType(somevar){
    return String(somevar.constructor)
            .split(/\({1}/)[0]
            .replace(/^\n/,'').substr(9);
}
var num = 43
    ,str = 'some string'
    ,obj = {}
    ,bool = false
    ,customObj = new (function SomeObj(){return true;})();

alert(whatType(num)); //=>Number
alert(whatType(str)); //=>String
alert(whatType(obj)); //=>Object
alert(whatType(bool)); //=>Boolean
alert(whatType(customObj)); //=>SomeObj

Based on the constructor property of variables you could also do:

function isType(variable,type){
 if ((typeof variable).match(/undefined|null/i) || 
       (type === Number && isNaN(variable)) ){
        return variable
  }
  return variable.constructor === type;
}
/** 
 * note: if 'variable' is null, undefined or NaN, isType returns 
 * the variable (so: null, undefined or NaN)
 */

alert(isType(num,Number); //=>true

Now alert(isType(customObj,SomeObj) returns false. But if SomeObj is a normal Constructor function, it returns true.

function SomeObj(){return true};
var customObj = new SomeObj;
alert(isType(customObj,SomeObj); //=>true