Javascript Undefined In Quotes

There are five falsey values in JavaScript: undefined is falsey. null is falsey. 0 is falsey. The empty string is falsey. NaN is falsey.

When you delete a property, you’re not just deleting the value of the property, you’re deleting the property itself. And, if you try to use fido.dogYears after deleting it, it will evaluate to undefined.

The type of undefined is undefined. Why?

What does it mean to assign the value null to a variable? How about “We intend to assign an object to this variable at some point, but we haven’t yet.” Now, if you’re scratching your head and saying “Hmm, why didn’t they just use undefined for that?” then you’re in good company. The answer comes from the very beginnings of JavaScript. The idea was to have one value for variables that haven’t been initialized to anything yet, and another that means the lack of an object. It isn’t pretty, and it’s a little redundant, but it is what it is at this point. Just

a return expression is not specified, then the return value will be undefined.

If a return expression is not specified, then the return value will be undefined.

A property value can be any JavaScript value except for undefined.

Attempting to retrieve values from undefined will throw a TypeError exception. This can be guarded against with the && operator: flight.equipment                              // undefined flight.equipment.model                        // throw "TypeError" flight.equipment && flight.equipment.model    // undefined

Here are the falsy values: false null undefined The empty string '' The number 0 The number NaN All other values are truthy, including true, the string 'false', and all objects.

Names A name is a letter optionally followed by one or more letters, digits, or underbars. A name cannot be one of these reserved words: abstract boolean break byte case catch char class const continue debugger default delete do double else enum export extends false final finally float for function goto if implements import in instanceof int interface long native new null package private protected public return short static super switch synchronized this throw throws transient true try typeof var volatile void while with Most of the reserved words in this list are not used in the language. The list does not include some words that should have been reserved but were not, such as undefined, NaN, and Infinity. It is not permitted to name a variable or parameter with a reserved word. Worse, it is not permitted to use a reserved word as the name of an object property in an object literal or following a dot in a refinement. Names are used for statements, variables, parameters, property names, operators, and labels.

JavaScript creates all local variables at the beginning of a function whether you declare them or not (this is called “hoisting” and we’ll come back to it later), but the variables are all undefined until they are assigned a value, which might not be what you want.

JavaScript’s global namespace is also exposed as a global object, which is accessible at the top of a program as the initial value of the this keyword. In web browsers, the global object is also bound to the global window variable. Adding or modifying global variables automatically updates the global object: this.foo; // undefined foo = "global foo"; this.foo; // "global foo

undefined and NaN are not constants. They are global variables, and you can change their values. That should not be possible, and yet it is. Don’t do it.

A function without a return statement returns undefined.