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:
NaN. See for yourself. console.log (null-undefined). The difference between null and undefined is NaN. (Note that this is an attempt at humour, before you flame me for misunderstanding the question.)
An undeclared variable does not exist in Javascript memory whereas a declared one does and can be set to undefined. The difference is caused by Javascript design confusion. If a variable's value is undefined then it should be deleted. Indeed falsy values like 0, "", null, and undefined, are all basically meaningless and identical. A lot of time is wasted on them. They are all nothing and non ...
There are 3 undefined in javascript. The global variable undefined, the value undefined and the type undefined. Even if the global variable undefined is overridden, the value undefined still exists. There are several ways to get it one of which is an empty argument to a function, another is a variable declaration without assigning anything:
The object.x === undefined should return true if x is unknown property. In chapter , Crockford writes the following: If you attempt to extract a value from an object, and if the object does not have a member with that name, it returns the undefined value instead. In addition to undefined, JavaScript has a similar value called null. They are so similar that == thinks they are equal. That ...
While literally using the keyword undefined, Boolean(undefined) works, trying that with an undefined variable doesn't work, and that is the whole point of doing the check for null or undefined.