Methods of judging null, undefined and nan in JS

  This article mainly introduces the method of determining null, undefined and nan in JS, and the friends who need it can refer to the following

wrote a str = "s" ++;  then appeared nan, looking for a while.   Collect data as follows to Judge:  1. Judge the undefined:  code as follows: <span style= "Font-size:small;" >var tmp = undefined;  if (typeof (tmp) = = "undefined") {  alert ("undefined"); }</span>  & nbsp Description: TypeOf returns a string of six possible: "Number", "string", "Boolean", "Object", "function", "undefined"   2. Judgment null:    The code is as follows: <span style= "Font-size:small;" >var tmp = null;  if (!tmp && typeof (TMP)!= "undefined" && tmp!=0) {  alert ("null"); } &L t;/span>    3. Judge nan:    code as follows: <span style= "Font-size:small;" >var tmp = 0/0;  if (isNaN (TMP)) {  alert ("NaN"); }</span>    Description: If NaN The result is false compared to any value (including itself), so you cannot use the = = or = = operator to determine whether a value is NaN.   Tip: the isNaN () function is commonly used to detect the results of parsefloat () and parseint () to determine whether they represent legitimate numbers. Of course, you can also use the isNaN () function to detect arithmetic errors, such as the case of dividing by 0.   4. Judge undefined and null:  code as follows: <span style= "Font-size:small;" >var tmp = undefined;  if (tmp== undefined)   {  alert ("null or undefined"); } </span>  code is as follows: <span style= "Font-size:small;" >var tmp = undefined;  if (tmp== null)   {  alert ("null or undefined"); }</span>    Description:null==undefined  <!--endfragment-->  5. Judge undefined, NULL, and nan:  code as follows: <span style= " Font-size:small; " >var tmp = null;  if (!tmp)   {  alert ("null or undefined or NaN"); }</span>    prompt: It is generally not so differentiated to use this enough.