Undefined learning and summarizing _javascript skills in JavaScript

This value is returned when you use a variable that is not declared, or when you use a variable that has been declared but not assigned, or when you use an object property that does not exist.

Although undefined and null are not the same, the = = operator regards both as equal, as

My.prop = null

If the property prop does not exist, or if it exists but the value is null, then the value of the expression is true.

When an undefined value is used in a Boolean environment, it is converted to false, and when it is used in a digital environment, it is converted to Nan, and when it is a user of a string environment, it is converted to "undefined"