![]() ![]() ![]() The typeof operator returns a string indicating the type of the unevaluated operand. You could even go and try to overwrite it – a word “undefined” is not among the reserved keywords. To check if the value is undefined in JavaScript, use the typeof operator. There is a global property called undefined, actually. The initial value of undefined is the primitive value undefined Simply put, undefined means a variable has been declared but has not yet. That is, it is a variable in global scope. Undefined is a property of the global object i.e., it is a variable in global scope. Many times we often get confused on what the difference between UNDEFINED and NULL is. undefined is a property of the global object. Both of them are falsy, watch out for some existing code, though (for example a library), that explicitly checks for undefined! It is a variable It seems like a good practice to set a variable to null if you mean to clear it, and not to undefined. undefined NaN These 7 values are the only falsy values in JavaScript. Undefined means that a variable has no value (or there is no property of given name) because it wasn’t assigned with a value so far (or was assigned with a value undefined). The null value means that it was purposely cleared by the programmer. Same goes for a return of a function, that does not, in fact, return anything. Undefined is a special value that lets the programmer know that a variable has not been given a value. To verify they are not equal, try using the strict equality operator: One way this error might occur in a real world example is if you try using a DOM element in your JavaScript before the element is loaded. Primitive value used when a variable has not been assigned a value Undefined is usually a variable that has not been assigned, while null means the value is blank. But what is it exactly? Definition of undefined It is a value ![]() Undefined is one of the reasons for confusion and can get under the skin of a developer. ![]()
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