JavaScript: The Definitive Guide, Fourth Edition

switch

An if statement causes a branch in the flow of a program’s execution. You can use multiple if statements, as in the previous section, to perform a multiway branch. However, this is not always the best solution, especially when all of the branches depend on the value of a single variable. In this case, it is wasteful to repeatedly check the value of the same variable in multiple if statements.

The switch statement (implemented in JavaScript 1.2 and standardized by ECMAScript v3) handles exactly this situation, and it does so more efficiently than repeated if statements. The JavaScript switch statement is quite similar to the switch statement in Java or C. The switch keyword is followed by an expression and a block of code, much like the if statement:

switch(expression) {
    statements
}

However, the full syntax of a switch statement is more complex than this. Various locations in the block of code are labeled with the case keyword followed by a value and a colon. When a switch executes, it computes the value of expression and then looks for a case label that matches that value. If it finds one, it starts executing the block of code at the first statement following the case label. If it does not find a case label with a matching value, it starts execution at the first statement following a special-case default: label. Or, if there is no default: label, it skips the block of code altogether.

switch is a confusing statement to explain; its operation becomes much clearer with ...