Js--if condition judgment

Now only special cases are mentioned:

1, a variable, such as N1=null

<script>        varnull;        Alert (N1); /* the value of the popup window is null */        if null) { /* If it is an if (N1) will not execute inside the function body *            /alert ("popup Window");        } </script>

2, a variable, such as N1, does not assign a value

<script>        var  N1;        Alert (N1); /* the value of the pop-up window is undefined */        if (typeof (n1) = = "undefined") {/* If it is an if (N1) or if (n1== "undefined") will not execute the inside function body *            /alert ("Popup Window" );        } </script>

3, a variable, such as n1,n1= ""

<script>        var n1 = "";        Alert (N1); /* the value of the popup window is empty (nothing is displayed, but the window is bounced) */        if (N1 = = "") { /* If it is an if (N1) will not execute inside the function body *            /alert ("popup Window");        } </script>

4, a variable, such as n1,n1= ""

<script>    var n1 = "   ";    Alert (N1); /* the value of the popup window is empty (nothing is displayed, but the window is bounced) */    if (N1) { /* If it is N1.trim (). length > 0 will not execute inside the function body *        /alert ("popup Window");    } </script>

5, a variable, such as N1,N1=0,N2 not assigned value

<script>        var n1 = 0;         var n2;         + N2); /* the value of the popup window is Nan */        if (IsNaN (n1 + n2)) { /* If an if (n1 + N2) will not execute inside the function body *            /alert ("popup Window");        } </script>

6, for the above situation, in addition to n1= "" assignment is a number of spaces this special case, others can be judged by the following code

if (!N1)

Note: Because of the above cases, direct write if (n1) or if (N1+N2) does not directly execute the code in the function body, so it is also possible to write this.

Js--if condition judgment