Use of javascript instanceof and typeof

Typeof is used to obtain the type of a variable. Generally, typeof can only return the following results: number, boolean, string, function, object, undefined. We can use typeof to obtain whether a variable exists, as shown in figure

If (typeof! = "Undefined") {}, instead of using if (a) because if a does not exist (not declared), an error occurs. For Array, null and other special objects use typeof to return all objects, which is the limitation of typeof.

If you want to obtain whether an object is an array or determine whether a variable is an instance of an object, you must use instanceof. Instanceof is an instance used to determine whether a variable is an Object, such as var a = new Array (); alert (a instanceof Array); true is returned, and alert (a instanceof Object) true is also returned because Array is a subclass of the object. For example, function test () {}; var a = new test (); alert (a instanceof test) returns true.

When talking about instanceof, We need to insert another question: function arguments. We may all think that arguments is an Array. However, if we use instaceof to test, we will find that arguments is not an Array object, although it looks very similar.

In JavaScript, The instanceof operator returns a Boolean value indicating whether the object is an instance of a specific class.
Usage:
Result = object instanceof class
Result is required. Any variable.
Object is required. Any object expression.
Class is required. Any defined object class.

Description
If an object is an instance of class, the instanceof operator returns true. If the object is not an instance of the specified class, or the object is null, false is returned.

Instanceof operator in JavaScript
The following example illustrates the usage of the instanceof operator.
Copy codeThe Code is as follows:
Function objTest (obj ){
Var I, t, s = ""; // create a variable.
T = new Array (); // create an Array.
T ["Date"] = Date; // fill in the array.
T ["Object"] = Object;
T ["Array"] = Array;
For (I in t)
{
If (obj instanceof t [I]) // check the obj class.
{
S + = "obj is an instance of" + I + "\ n ";
}
Else
{
S + = "obj is not an instance of" + I + "\ n ";
}
}
Return (s); // return a string.
}

Var obj = new Date ();
Response. write (objTest (obj ));