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

A switch statement first evaluates its expression. It then looks for the first case clause whose expression evaluates to the same value as the result of the input expression (using strict comparison, ===) and transfers control to that clause, executing the associated statements. (If multiple cases match the provided value, the first case that matches is selected, even if the cases are not equal to each other.) If no matching case clause is found, the program looks for the optional default clause, and if found, transfers control to that clause, executing the associated statements. If no default clause is found, the program continues execution at the statement following the end of switch. By convention, the default clause is the last clause, but it does not need to be so.
Read more: MDN: Switch

switch-statement strict-comparison

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

break-keyword