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I wanted to check whether the variable is defined or not. For example, the following throws a not-defined error
alert( x );
How can I catch this error?
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If it is undefined, it will not be equal to a string that contains the characters "undefined", as the string is not undefined. You can check the type of the variable: if (typeof(something) != "undefined") ...
To check if a variable is equal to null or undefined , use the loose equality (==) operator. For example, age == null returns true if the variable age is null or undefined .
myVariable is declared and not yet assigned with a value. Accessing the variable evaluates to undefined . An efficient approach to solve the troubles of uninitialized variables is whenever possible assign an initial value. The less the variable exists in an uninitialized state, the better.
A variable that has not been assigned a value is of type undefined . A method or statement also returns undefined if the variable that is being evaluated does not have an assigned value. A function returns undefined if a value was not returned .
In JavaScript, null is an object. There's another value for things that don't exist, undefined. The DOM returns null for almost all cases where it fails to find some structure in the document, but in JavaScript itself undefined is the value used.
Second, no, there is not a direct equivalent. If you really want to check for specifically for null, do:
if (yourvar === null) // Does not execute if yourvar is `undefined`
If you want to check if a variable exists, that can only be done with try/catch, since typeof will treat an undeclared variable and a variable declared with the value of undefined as equivalent.
But, to check if a variable is declared and is not undefined:
if (yourvar !== undefined) // Any scope
Previously, it was necessary to use the typeof operator to check for undefined safely, because it was possible to reassign undefined just like a variable. The old way looked like this:
if (typeof yourvar !== 'undefined') // Any scope
The issue of undefined being re-assignable was fixed in ECMAScript 5, which was released in 2009. You can now safely use === and !== to test for undefined without using typeof as undefined has been read-only for some time.
If you want to know if a member exists independent but don't care what its value is:
if ('membername' in object) // With inheritance
if (object.hasOwnProperty('membername')) // Without inheritance
If you want to to know whether a variable is truthy:
if (yourvar)
Source
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The only way to truly test if a variable is undefined is to do the following. Remember, undefined is an object in JavaScript.
if (typeof someVar === 'undefined') {
// Your variable is undefined
}
Some of the other solutions in this thread will lead you to believe a variable is undefined even though it has been defined (with a value of NULL or 0, for instance).
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