JQuery/Javascript: check if var exists

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How can I check whether a variable is defined in JavaScript?
Is there a standard function to check for null, undefined, or blank variables in JavaScript?

I have a script that occurs in two parts.

The first part sets up a var:

var pagetype = "textpage";

The 2nd part is a simple if statement:

if(pagetype == "textpage") {
//do something
};

Now the 2nd part, the if statement, appears on all pages of my site. But the first part, where the var is declared, only appears on some of my pages.

On the pages without the var I naturally get this error:

Uncaught ReferenceError: pagetype is not defined

So my question is: is there a way with JavaScript or JQ to detect if a var exists at all (not just if it has data assigned to it)?

I am imagining I would just use another if statment, eg:

if ("a var called pagetypes exists")....
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最佳答案

I suspect there are many answers like this on SO but here you go:

if ( typeof pagetype !== 'undefined' && pagetype == 'textpage' ) {
...
}
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You can use typeof:

if (typeof pagetype === 'undefined') {
// pagetype doesn't exist
}
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To test for existence there are two methods.

a. "property" in object

This method checks the prototype chain for existence of the property.

b. object.hasOwnProperty( "property" )

This method does not go up the prototype chain to check existence of the property, it must exist in the object you are calling the method on.

var x; // variable declared in global scope and now exists


"x" in window; // true
window.hasOwnProperty( "x" ); //true

If we were testing using the following expression then it would return false

typeof x !== 'undefined'; // false
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Before each of your conditional statements, you could do something like this:

var pagetype = pagetype || false;
if (pagetype === 'something') {
//do stuff
}
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It is impossible to determine whether a variable has been declared or not other than using try..catch to cause an error if it hasn't been declared. Test like:

if (typeof varName == 'undefined')

do not tell you if varName is a variable in scope, only that testing with typeof returned undefined. e.g.

var foo;
typeof foo == 'undefined'; // true
typeof bar == 'undefined'; // true

In the above, you can't tell that foo was declared but bar wasn't. You can test for global variables using in:

var global = this;
...
'bar' in global;  // false

But the global object is the only variable object* you can access, you can't access the variable object of any other execution context.

The solution is to always declare variables in an appropriate context.

  • The global object isn't really a variable object, it just has properties that match global variables and provide access to them so it just appears to be one.
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For your case, and 99.9% of all others elclanrs answer is correct.

But because undefined is a valid value, if someone were to test for an uninitialized variable

var pagetype; //== undefined
if (typeof pagetype === 'undefined') //true

the only 100% reliable way to determine if a var exists is to catch the exception;

var exists = false;
try { pagetype; exists = true;} catch(e) {}
if (exists && ...) {}

But I would never write it this way


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