null, undefined, NaN in JavaScript

1. Judge whether each other is equal

function Btnclick () {alert (null = = NULL);//true alert (undefined==undefined);//true alert (Nan==nan);//false}

Summarize:

(1) Two null is equal

(2) Two undefined are equal

(3) Two Nan is unequal

function Btnclick () {alert (null = = undefined),//true alert (null = = = undefined),//false alert (null = = NaN);//false alert (undefined = = NaN);//false}

Summarize:

(1) null and undefined are equal by using "= =", while "= = =" is not equal

(2) null and Nan are not equal

(3) undefined and Nan are not equal

function Btnclick () {alert (typeof (null));//object alert (typeof (undefined));//undefined}

Summarize:

(1) The result of typeof (null) is "Object"

(2) The result of typeof (undefined) is "undefined"

2. Determine if null, undefined, NaN

Use = =, = = =, isxxx, typeof, Instanceof

2.1. Judge whether it is undefined

the right approach :

var tmp = Undefined;function Btnclick () {if (typeof (tmp) = = = "undefined") {alert ("TMP is undefined!"); }}

Or

var tmp = NULL;    function Btnclick () {if (tmp = = = undefined) {alert ("TMP is undefined!"); }}

the wrong approach :

var tmp = undefined;    Null,undefinedfunction Btnclick () {if (tmp = = undefined) {alert ("TMP is undefined!"); }}

Analysis: If you use tmp==undefined, the value of TMP will be determined to be true regardless of whether it is null or undefined.

2.2. Determine if NULL

the right approach :

var tmp = NULL; Null,nan,undefinedfunction Btnclick () {if (!tmp && typeof (TMP)! = "undefined" &&!isnan (tmp)) {A    Lert ("TMP is null!"); }}

Or

var tmp = NULL; Null,nan,undefinedfunction Btnclick () {if (Tmp==null && typeof (TMP)! = "undefined") {alert ("TMP is null!    "); }}

2.3. Determine if it is Nan

The right approach:

var tmp = NaN; Null,nan,undefinedfunction Btnclick () {if (IsNaN (TMP) && typeof (TMP)! = "undefined") {alert ("TMP is nan!    "); }}

2.4. Determine if null or undefined

The right approach:

var tmp = NULL;    Null,nan,undefinedfunction Btnclick () {if (TMP = = null) {alert ("TMP is null or undefined!"); }}

Or

var tmp = undefined;    Null,nan,undefinedfunction Btnclick () {if (tmp = = undefined) {alert ("TMP is null or undefined!"); }}

2.5. Judge whether it is undefined or Nan

Correct practice:

var tmp = NaN;    Null,nan,undefinedfunction Btnclick () {if (IsNaN (TMP)) {alert ("TMP is NaN or undefined!"); }}

2.6. Determine if null or undefined or Nan

The right approach:

var tmp = NULL;    Null,nan,undefinedfunction Btnclick () {if (!tmp) {alert ("TMP is null or undefined or nan!"); }}

null, undefined, NaN in JavaScript