JavaScript_Succinctly

Chapter 8 Undefined
Conceptual overview of the undefined value
The undefined value is used by JavaScript in two slightly different ways.
The first way it's used is to indicate that a declared variable (e.g., var foo) has no
assigned value. The second way it's used is to indicate that an object property you’re
trying to access is not defined (i.e. it has not even been named), and is not found in the
prototype chain.
In the following sample, I examine both usages of undefined by JavaScript.
Sample: sample62.html
var initializedVariable; // Declare variable.
console.log(initializedVariable); // Logs undefined.
console.log(typeof initializedVariable); // Confirm that JavaScript
returns undefined.
var foo = {};
console.log(foo.bar); // Logs undefined, no bar property in foo object.
console.log(typeof foo.bar); // Confirm that JavaScript returns
undefined.
Notes
It is considered good practice to allow JavaScript alone to use undefined. You should
never find yourself setting a value to undefined, as in foo = undefined. Instead, null
should be used if you are specifying that a property or variable value is not available.
JavaScript ECMA-262 Edition 3 (and later) declares the undefined
variable in the global scope
Unlike previous versions, JavaScript ECMA-262 Edition 3 (and later) has a global
variable called undefined declared in the global scope. Because the variable is
declared and not assigned a value, the undefined variable is set to undefined.
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