Js determines the undefined type
When the page opens today with the showModalDialog and returns a value. The return value is undefined when a page opens by clicking the close button or clicking close on the browser So wise wise & cake;
Var reValue = window. ShowModalDialog (” ”, "",” ”); If (reValue = = undefined) { Alert (” undefined ”); }
It was found that the judgment could not be made, so typeof was used to check the data at last
**Methods: **If (typeof(reValue) == “undefined”) { Alert (” undefined ”); } Typeof returns a string, with six possibilities: “number”, “string”, “Boolean”, “object”, “function”, “undefined”
Undefined,null,NaN in js
**1. Type analysis: **Js have undefined the data types, number, Boolean, string, object and so on five, four for the primitive type, before 5 kinds of reference types. Var a1. Var a2 = true; Var a3 = 1; Var a4 = “Hello”; Var a5 = new Object(); Var a6 = null; Var a7 = NaN; Var a8 = undefined; Alert (typeof a); / / show “undefined” Alert (typeof a1); / / show “undefined” Alert (typeof a2); / / show “Boolean” Alert (typeof a3); / / show the “number” Alert (typeof a4); / / show “string” Alert (typeof a5); / / show “object” Alert (typeof a6); / / show “object” Alert (typeof a7); / / show the “number” Alert (typeof a8); / / show “undefined”
From the above code, you can see that undefined and undefined values are undefined, null is a special object, and NaN is a special number.
**2. Comparison operations **Var a1. //a1 is undefined Var a2 = null; Var a3 = NaN; Alert (a1 = = a2); / / show “true” Alert (a1! = a2); / / show “false” Alert (a1 = = a3); / / show “false” Alert (a1! = a3); / / show “true” Alert (a2 = = a3); / / show “false” Alert (a2! = a3); / / show “true” Alert (a3 = = a3); / / show “false” Alert (a3! = a3); / / show “true”
From the above code we can conclude that :(1) undefined is equal to null; (2) NaN is not equal to any value, nor is it equal to itself.
JavaScript undefined attribute
**Definition and usage **The undefined property is used to hold the undefined value of JavaScript.
**grammar **undefined
**instructions **The for/in loop cannot be used to enumerate the undefined property, nor can the delete operator be used to delete it. Undefined is not a constant, you can set it to something else. Undefined is also returned when an attempt is made to read a property of a nonexistent object.
**Hints and comments **Tip: you can only use the === operation to test whether a value is undefined, because the == operator considers an undefined value to be null. Note: null means no value, and undefined means an undeclared variable, or a declared but unassigned variable, or a nonexistent object property.
**The instance **In this example, we will detect the undefined one of the two variables: < The script type = “text/javascript” > Var t1 = "" Var t2 If (t1===undefined) {document.write(“t1 is undefined”)} If (t2===undefined) {document.write(“t2 is undefined”)} < / script>
Output: T2 is undefined