case

In JavaScript, the case keyword starts a clause inside a switch statement and gives a value to compare against the switch expression. When the switch statement runs, it evaluates the case expressions in source order and jumps to the first one that’s strictly equal (===) to its value. If none matches, it jumps to the default clause, if there is one.

Because matching is strict, the string "1" doesn’t match case 1:, and case NaN: never matches. Each case expression is evaluated only if no earlier clause matched.

A case clause has no end of its own. After a match, execution falls through into the following clauses until a break, return, or throw leaves the statement. All clauses also share one block scope, so a let or const name declared in one case clashes with the same name in another. Wrapping the clause’s statements in braces gives it its own block.

switch (<expression>) {
  case <value1>:
    [<statements>]
  [case <value2>:
    [<statements>]]
  [default:
    [<statements>]]
}

> function describe(value) {
…   switch (value) {
…     case 1:
…       return "the number 1";
…     case "a":
…     case "b":
…       return "an early letter";
…     default:
…       return "something else";
…   }
… }
undefined

> describe(1)
'the number 1'
> describe("1")
'something else'

> describe("b")
'an early letter'

> switch (2) {
…   case 1:
…     console.log("one");
…   case 2:
…     console.log("two");
…   case 3:
…     console.log("three");
…     break;
…   case 4:
…     console.log("four");
… }
two
three
undefined