The switch statement evaluates an expression, matching the expression's value to a case clause, and executes statements associated with that case.

Syntax

switch (expression) {
  case value1:
    //Statements executed when the result of expression matches value1
    [break;]
  case value2:
    //Statements executed when the result of expression matches value2
    [break;]
  ...
  case valueN:
    //Statements executed when the result of expression matches valueN
    [break;]
  default:
    //Statements executed when none of the values match the value of the expression
    [break;]
}

expression

An expression whose result is matched against each case clause.

case valueN

A case clause used to match against expression.

Read more: MDN: Switch

The Switch Statement

The optional break statement associated with each case label ensures that the program breaks out of switch once the matched statement is executed and continues execution at the statement following switch. If break is omitted, the program continues execution at the next statement in the switch statement.

What happens if I forgot a break?

If you forget a break then script will run from the case where criteria is met, and will run the case after that regardless if criteria was met. See example here:

var foo = 0;
switch (foo) {
  case -1:
    console.log('negative 1');
    break;
  case 0: 
    console.log(0);
    // NOTE: the forgotten break 
  case 1:     
    console.log(1);
    break; //  break 
  case 2:
    console.log(2);
    break;
}

Console output:

01

Read more: MDN: Switch

switch-statement break-keyword

Multi-case - single operation

This method takes advantage of the fact that if there is no break below a case statement it will continue to execute the next case statement regardless if the case meets the criteria.
This is an example of a single operation sequential switch statement, where four different values perform exactly the same.

var Animal = 'Giraffe';
switch (Animal) {
  case 'Cow':
  case 'Giraffe':
  case 'Dog':
  case 'Pig':
    console.log('This animal will go on Noah\'s Ark.');
    break;
  case 'Dinosaur':
  default:
    console.log('This animal will not.');
}

Read more: Stackoverflow: Switch statement multiple cases in JavaScript

multi-case-single-operation multi-case