![]() This can often be the case when choosing to go with function expressions rather than declarations and neglect to do what is required to keep that from happening, or it could just be a simple typo. If I attempt to call an object property that I expect is a function, but turns out to be undefined, that can result in an Error that is the result of calling undefined. However an object key and a variable can be both declared, and intensionally assigned the value undefined bu using the undefined keyword, or preforming any kind of action that will result in the value of undefined being set to the variable or object property. In addition if I attempt to access an object property value that is not defined, then the result is undefined. For one thing the undefined value is the default value for variables that are declared, but do not have any value assigned to them. b is defined as a null-value.In javaScript undefined is a value that comes up often when working with various code examples, and projects. The difference between the two is perhaps a bit more clear through code: let a Ĭonsole.log(a) // undefined let b = null Ī is undefined - it's not assigned to anything, and there's no clear definition as to what it really is. null is a variable that is defined but is missing a value. ![]() undefined is a variable that refers to something that doesn't exist, and the variable isn't defined to be anything.Though, there is a difference between them: Undefined and null variables often go hand-in-hand, and some use the terms interchangeably. In this short guide, we'll take a look at how to check if a variable is undefined or null in JavaScript. Whether we lose a reference through side-effects, forget to assign a reference variable to an object in memory, or we get an empty response from another resource, database or API - we have to deal with undefined and null values all the time. Undefined and null values sneak their way into code flow all the time.
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