Why aren't they keywords? What are they?

true, false, null

Update Quick Answer

These are reserved words but they are not keywords.

Small technical distinction verified by spec - ES3 and ES5

Why aren't they keywords? What are they?

true, false, null

Update Quick Answer

These are reserved words but they are not keywords.

Small technical distinction verified by spec - ES3 and ES5

• javascript
• keyword

• 1 What makes you say they aren't reserved words? true = 6 will never work, that kind of makes it a reserved word. – Niet the Dark Absol Commented Sep 5, 2012 at 20:08
• sorry...they are not keywords. – user656925 Commented Sep 11, 2012 at 22:07

4 Answers 4

They are boolean literals. From the specification:

BooleanLiteral ::
   true
   false

• The value of the Boolean literal true is a value of the Boolean type, namely true.
• The value of the Boolean literal false is a value of the Boolean type, namely false.

It is similar to how 10 is a numeric literal or 'foo' is a string literal.

Reserved words includes keywords and literals. The words true and false are reserved words, but they are not keywords. The following are keywords:

break    do       instanceof typeof
case     else     new        var
catch    finally  return     void
continue for      switch     while
debugger function this       with
default  if       throw
delete   in       try

Notice that true and false don't appear in this list.

I think your confusion es from not realising that the two terms keyword and reserved word are not the same. Every keyword is a reserved word, but not every reserved word is a keyword.

Actually true and false are reserved words in Javascript, from:

http://ecma-international/ecma-262/5.1/#sec-7.6.1

A reserved word is an IdentifierName that cannot be used as an Identifier.

Syntax
    ReservedWord ::
        Keyword
        FutureReservedWord
        NullLiteral
        BooleanLiteral

and in

http://ecma-international/ecma-262/5.1/#sec-7.8.2

you can read:

Boolean Literals

Syntax
    BooleanLiteral ::
        true
        false

Mozilla Documentation

Additionally, the literals null, true, and false are reserved in ECMAScript for their normal uses.

They actually do appear to be "reserved" for usage, but I have no clue why they are not listed as a reserved word.

With non object javascript you just write

if( sami.value = true) //Noticed i didn't put ==

if it is object The Boolean object represents two values: "true" or "false".

The following code creates a Boolean object called myBoolean:

var myBoolean=new Boolean();

If the Boolean object has no initial value, or if the passed value is one of the following:

0
-0
null
""
false
undefined
NaN

the object is set to false. For any other value it is set to true (even with the string "false")!