2.13-The-switch-statement.md

2.13 The "switch" statement

A switch statement can replace multiple if checks.

It gives a more descriptive way to compare a value with multiple variants.

The syntax

The switch has one or more case blocks and an optional default.

It looks like this:

switch(x) {
  case 'value1':  // if (x === 'value1')
    ...
    [break]

  case 'value2':  // if (x === 'value2')
    ...
    [break]

  default:
    ...
    [break]
}

The value of x is checked for a strict equality to the value from the first case (that is, value1) then to the second (value2) and so on.

If the equality is found, switch starts to execute the code starting from the corresponding case, until the nearest break (or until the end of switch).

If no case is matched then the default code is executed (if it exists).

An example

An example of switch (the executed code is highlighted):

let a = 2 + 2;

switch (a) {
  case 3:
    alert( 'Too small' );
    break;
  case 4:
    alert( 'Exactly!' );
    break;
  case 5:
    alert( 'Too large' );
    break;
  default:
    alert( "I don't know such values" );
}

Here the switch starts to compare a from the first case variant that is 3. The match fails.

Then 4. That’s a match, so the execution starts from case 4 until the nearest break.

If there is no break then the execution continues with the next case without any checks.

An example without break:

let a = 2 + 2;

switch (a) {
  case 3:
    alert( 'Too small' );
  case 4:
    alert( 'Exactly!' );
  case 5:
    alert( 'Too big' );
  default:
    alert( "I don't know such values" );
}

In the example above we’ll see sequential execution of three alerts:

alert( 'Exactly!' );
alert( 'Too big' );
alert( "I don't know such values" );

---

Any expression can be a switch/case argument

Both switch and case allow arbitrary expressions.

For example:

let a = "1";
let b = 0;

switch (+a) {
  case b + 1:
    alert("this runs, because +a is 1, exactly equals b+1");
    break;

  default:
    alert("this doesn't run");
}

Here +a gives 1, that’s compared with b + 1 in case, and the corresponding code is executed.

---

Grouping of “case”

Several variants of case which share the same code can be grouped.

For example, if we want the same code to run for case 3 and case 5:

let a = 2 + 2;

switch (a) {
  case 4:
    alert('Right!');
    break;

  case 3:                    // (*) grouped two cases
  case 5:
    alert('Wrong!');
    alert("Why don't you take a math class?");
    break;

  default:
    alert('The result is strange. Really.');
}

Now both 3 and 5 show the same message.

The ability to “group” cases is a side-effect of how switch/case works without break. Here the execution of case 3 starts from the line (*) and goes through case 5, because there’s no break.

Type matters

Let’s emphasize that the equality check is always strict. The values must be of the same type to match.

For example, let’s consider the code:

let arg = prompt("Enter a value?")
switch (arg) {
  case '0':
  case '1':
    alert( 'One or zero' );
    break;

  case '2':
    alert( 'Two' );
    break;

  case 3:
    alert( 'Never executes!' );
    break;
  default:
    alert( 'An unknown value' )
}

1. For 0, 1, the first alert runs.

2. For 2 the second alert runs.

3. But for 3, the result of the prompt is a string "3", which is not strictly equal === to the number 3. So we’ve got a dead code in case 3! The default variant will execute.