A switch statement can replace multiple if checks.
It gives a more descriptive way to compare a value with multiple variants.
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 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 big' );
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.
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 = 3;
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.
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' );
}For 0, 1, the first alert runs.
For 2 the second alert runs.
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.
importance: 5
Write the code using if..else which would correspond to the following switch:
switch (browser) {
case 'Edge':
alert( "You've got the Edge!" );
break;
case 'Chrome':
case 'Firefox':
case 'Safari':
case 'Opera':
alert( 'Okay we support these browsers too' );
break;
default:
alert( 'We hope that this page looks ok!' );
}To precisely match the functionality of switch, the if must use a strict comparison '==='.
For given strings though, a simple '==' works too.
if(browser == 'Edge') {
alert("You've got the Edge!");
} else if (browser == 'Chrome'
|| browser == 'Firefox'
|| browser == 'Safari'
|| browser == 'Opera') {
alert( 'Okay we support these browsers too' );
} else {
alert( 'We hope that this page looks ok!' );
}Please note: the construct browser == 'Chrome' || browser == 'Firefox' … is split into multiple lines for better readability.
But the switch construct is still cleaner and more descriptive.
importance: 4
Rewrite the code below using a single switch statement:
let a = +prompt('a?', '');
if (a == 0) {
alert( 0 );
}
if (a == 1) {
alert( 1 );
}
if (a == 2 || a == 3) {
alert( '2,3' );
}The first two checks turn into two case. The third check is split into two cases:
let a = +prompt('a?', '');
switch (a) {
case 0:
alert( 0 );
break;
case 1:
alert( 1 );
break;
case 2:
case 3:
alert( '2,3' );
break;
}Please note: the break at the bottom is not required. But we put it to make the code future-proof.
In the future, there is a chance that we’d want to add one more case, for example case 4. And if we forget to add a break before it, at the end of case 3, there will be an error. So that’s a kind of self-insurance.