JavaScript: The Definitive Guide, 6th Edition

Boolean Values

A boolean value represents truth or falsehood, on or off, yes or no. There are only two possible values of this type. The reserved words true and false evaluate to these two values.

Boolean values are generally the result of comparisons you make in your JavaScript programs. For example:

a == 4

This code tests to see whether the value of the variable a is equal to the number 4. If it is, the result of this comparison is the boolean value true. If a is not equal to 4, the result of the comparison is false.

Boolean values are commonly used in JavaScript control structures. For example, the if/else statement in JavaScript performs one action if a boolean value is true and another action if the value is false. You usually combine a comparison that creates a boolean value directly with a statement that uses it. The result looks like this:

if (a == 4)
  b = b + 1;
else
  a = a + 1;

This code checks whether a equals 4. If so, it adds 1 to b; otherwise, it adds 1 to a.

As we’ll discuss in Type Conversions, any JavaScript value can be converted to a boolean value. The following values convert to, and therefore work like, false:

undefined
null
0
-0
NaN
""  // the empty string

All other values, including all objects (and arrays) convert to, and work like, true. false, and the six values that convert to it, are sometimes called falsy values, and all other values are called truthy. Any time JavaScript expects a boolean value, a falsy value works like false and a truthy value works like true ...