The "switch" statement

The syntax

如果 switch 的變數跟 case 一樣就會執行，如果沒有 break 會繼續執行下一個 case。

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

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

  default:
    ...
    [break]
}

An example

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" );
}

// can be expression
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");
}

Grouping of “case”

執行一樣的 code，可以寫成巢狀。

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.');
}

Type matters

因為用 === 判斷 code 是否執行，所以值的類型很重要。

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' );
}