JavaScript: The Definitive Guide, 6th Edition

Conditionals

Conditional statements execute or skip other statements depending on the value of a specified expression. These statements are the decision points of your code, and they are also sometimes known as “branches.” If you imagine a JavaScript interpreter following a path through your code, the conditional statements are the places where the code branches into two or more paths and the interpreter must choose which path to follow.

The subsections below explain JavaScript’s basic conditional, the if/else statement, and also cover switch, a more complicated multiway branch statement.

if

The if statement is the fundamental control statement that allows JavaScript to make decisions, or, more precisely, to execute statements conditionally. This statement has two forms. The first is:

if (expression)
    statement

In this form, expression is evaluated. If the resulting value is truthy, statement is executed. If expression is falsy, statement is not executed. (See Boolean Values for a definition of truthy and falsy values.) For example:

if (username == null)       // If username is null or undefined,
    username = "John Doe";  // define it

Or similarly:

// If username is null, undefined, false, 0, "", or NaN, give it a new value
if (!username) username = "John Doe";

Note that the parentheses around the expression are a required part of the syntax for the if statement.

JavaScript syntax requires a single statement after the if keyword and parenthesized expression, but you can use a statement block to combine ...