For undeclared variables, typeof foo will return the string literal “undefined”, whereas the identity check foo === undefined would trigger the error “foo is not defined”. For local variables (which you know are declared somewhere), no such error would occur, hence the identity check.
You want the typeof operator. Specifically: if (typeof variable !== ‘undefined’) { // the variable is defined }
I think the most efficient way to test for “value is null or undefined” is if ( some_variable == null ){ // some_variable is either null or undefined } So these two lines are equivalent: if ( typeof(some_variable) !== “undefined” && some_variable !== null ) {} if ( some_variable != null ) {} Note 1 … Read more
How do I check a variable if it’s null or undefined… Is the variable null: if (a === null) // or if (a == null) // but see note below …but note the latter will also be true if a is undefined. Is it undefined: if (typeof a === “undefined”) // or if (a === … Read more
The delete operator removes a property from an object. It cannot remove a variable. So the answer to the question depends on how the global variable or property is defined. (1) If it is created with var, it cannot be deleted. For example: var g_a = 1; //create with var, g_a is a variable delete … Read more
(name is undefined) You: What is name? (*) JavaScript: name? What’s a name? I don’t know what you’re talking about. You haven’t ever mentioned any name before. Are you seeing some other scripting language on the (client-)side? name = null; You: What is name? JavaScript: I don’t know. In short; undefined is where no notion … Read more
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 … Read more
undefined means a variable has been declared but has not yet been assigned a value: var testVar; alert(testVar); //shows undefined alert(typeof testVar); //shows undefined null is an assignment value. It can be assigned to a variable as a representation of no value: var testVar = null; alert(testVar); //shows null alert(typeof testVar); //shows object From the … Read more
You want the typeof operator. Specifically: if (typeof variable !== ‘undefined’) { // the variable is defined }
You can use the qualities of the abstract equality operator to do this: if (variable == null){ // your code here. } Because null == undefined is true, the above code will catch both null and undefined.