switch statements

switch and if

Before introducing the syntax, the original lesson compares switch with a sequence of if statements. It describes an instruction as a command the CPU reads from memory and argues that repeated if checks require more instructions, while a switch checks its input once. On that basis, it recommends switch when there are many cases.

Editorial correction: that performance explanation is too general. JavaScript engine optimizations, the case values, and the surrounding code determine the actual cost. A switch is not guaranteed to use fewer instructions or be faster. Choose the form that clearly expresses the logic, and measure when performance matters.

The structure of switch

A switch usually contains one or more case clauses and may include a default. Think of default as the counterpart to a final else: it runs when no case matches.

Put the value to inspect inside the parentheses after switch. Each case supplies a value to match and statements to run. A break ordinarily ends the selected branch and exits the switch.

If you omit break, execution can continue into the next case's statements. This is called fall-through.

Walk through the example

In the original example, a is 4:

• If a is 1, print "a = 1" and exit.
• If a is 2, print "a = 2" and exit.
• If a is 3, print "a = 3" and exit.
• Otherwise, the default prints "not 1, 2, 3" and exits.

Open the original switch example on CodePen.

The next lesson covers loops.