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5.4.1 if

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


5.4.1 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 §3.3 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 multiple statements into one. So the if statement might also look like this:

if (!address) {

address = "";

message = "Please specify a mailing address.";

}

The second form of the if statement introduces an else clause that is executed when expression is false. Its syntax is: if (expression) statement1

else

statement2

This form of the statement executes statement1 if expression is truthy and executes statement2 if expression is falsy. For example:

if (n == 1)

console.log("You have 1 new message.");

else

console.log("You have " + n + " new messages.");

When you have nested if statements with else clauses, some caution is required to ensure that the else clause goes with the appropriate if statement. Consider the following lines:

i = j = 1;

k = 2;

if (i == j)

if (j == k)

console.log("i equals k");

else

console.log("i doesn't equal j"); // WRONG!!

In this example, the inner ifstatement forms the single statement allowed by the syntax of the outer if statement. Unfortunately, it is not clear (except from the hint given by the indentation) which if the else goes with. And in this example, the indentation is wrong, because a JavaScript interpreter actually interprets the previous example as:

if (i == j) { if (j == k) console.log("i equals k"); else console.log("i doesn't equal j"); // OOPS! }

The rule in JavaScript (as in most programming languages) is that by default an else clause is part of the nearest if statement. To make this example less ambiguous and easier to read, understand, maintain, and debug, you should use curly braces:

if (i == j) {

if (j == k) {

console.log("i equals k");

}

}

else { // What a difference the location of a curly brace makes!

5.4 Conditionals | 93

console.log("i doesn't equal j"); }

Although it is not the style used in this book, many programmers make a habit of enclosing the bodies of if and else statements (as well as other compound statements, such as while loops) within curly braces, even when the body consists of only a single statement. Doing so consistently can prevent the sort of problem just shown.

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