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...

This article listed various way to implement "foreach" in JavaScript and introduced very clear. If you want to find simplified explanation, you can read this article instead. I personally prefer to use "for-in" loop, let's see examples on array and object. Using for-in to loop through an array var arr = ['val1','val2','val3']; for (var key in arr) { console.log(key+' => '+arr[key]); } //0 => val1 //1 => val2 //2 => val3  Using for-in to loop through an object var obj = {'key1':'val1','key2':'val2','key3':'val3'}; for (var key in obj) { console.log(key+' => '+obj[key]); } //key1 => val1 //key2 => val2 //key3 => val3  Using for-in to loop through objects inside an array var arr = [ {'key11':'val11','key12':'val12','key13':'val13'}, {'key21':'val21','key22...