CyL's nOtE

In most JavaScript cases, evaluating variable is undefined, null, or NaN is very common. Their definition are shown below: - undefined : a declared variable which is not assigned value. - null : object value which means "no value". - NaN : "Not-a-Number" value Example for checking "undefined" var a = 1; var b; console.log(a===undefined); //false console.log(b===undefined); //true //console.log(c===undefined); //Error occurs due to "c" is not declared yet. console.log(typeof c === undefined); //false ("typeof" will return string value) console.log(typeof c === 'undefined'); //true Example for checking "null" var a = null; var b; console.log(a===null); //true console.log(b===null); //false console.log(typeof a); //object Example for checking "NaN" var a = NaN; var b; var c = 10; var d = '10'; var e = 'string10'; var f = '10string'; var g = parseInt('10'); v