![]() ![]() ![]() Safari: can't find variable foo Still not clear…”unresolvable reference”? As we will see none of these messages is particularly enlightening: alert(foo) Note how the message syntax varies across browser. If you’re interested check the reading list at the end of this article) (There are other cases where a ReferenceError will be thrown, most notably when running in ECMA 5 Strict mode. In practical terms, this means a ReferenceError will be thrown when JavaScript attempts to get the value of an unresolvable reference. So what’s a ReferenceError?Ī ReferenceError indicates that an invalid reference value has been detected (ECMA 5 15.11.6.3) The only similarity between undefined and null is they both coerce to false. Yes, generally well understood but worth re-stating: undefined is distinct from null which is also a primitive value representing the intentional absence of a value. Needless to say, re-assigning values to the undefined variable is very bad practice, and in fact its not allowed by ECMA 5 (though amongst the current set of full browser releases, only Safari enforces this). typeof undefined //"undefined"į = undefined //re-assigning to undefined (variable)Īs of ECMA 3, its value can be reassigned : undefined = "washing machine" //assign a string to undefined (variable) For consistency I’m always going to call it a variable in this article. Undefined (variable) is a global property whose initial value is undefined (value), Since its a global property we can also access it as a variable. The value of an unsupplied function argument is undefined. A function without a return statement, or a function with an empty return statement returns undefined. Any property that has not been assigned a value, assumes the undefined value. Undefined (value) is a primitive and is the sole value of the Undefined type. Undefined (type) is a built-in JavaScript type. In JavaScript there is Undefined (type), undefined (value) and undefined (variable). If you’re not already familiar with the difference between variables and properties in JavaScript (including the internal VariableObject) now might be a good time to check out my previous posting. ![]() This is my attempt to straighten things out a little. In particular, trying to understand ReferenceErrors (“x is not defined”) and how best to code against them can be frustrating. Compared to other languages, JavaScript’s concept of undefined is a little confusing. ![]()
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