else if (JavaScript: The Definitive Guide, 4th Edition)

6.4. else if

We've seen that the if/else statement is useful for testing a condition and executing one of two pieces of code, depending on the outcome. But what about when we need to execute one of many pieces of code? One way to do this is with an else if statement. else if is not really a JavaScript statement, but simply a frequently used programming idiom that results when repeated if/else statements are used:

if (n == 1) {
    // Execute code block #1
}
else if (n == 2) {
    // Execute code block #2
}
else if (n == 3) {
    // Execute code block #3
}
else {
    // If all else fails, execute block #4
}

There is nothing special about this code. It is just a series of if statements, where each if is part of the else clause of the previous statement. Using the else if idiom is preferable to, and more legible than, writing these statements out in their syntactically equivalent fully nested form:

if (n == 1) {
    // Execute code block #1
}
else {
    if (n == 2) {
        // Execute code block #2
    }
    else {
        if (n == 3) {
            // Execute code block #3
        }
        else {
            // If all else fails, execute block #4
        }
    }
}