Conditional statements¶

Conditional statements are mechanisms built into the language that execute the selected code, depending on whether the value of an expression is true or false. JavaScript offers the following possibilities:

• if
• if - else
• else if
• switch

if¶

These instructions have a similar syntax to that you already know from Java. I.e., after the keyword if, the appropriate condition is given in parentheses. If it evaluates to true, the code from the inner block (inside the { and }) will be executed, otherwise, it is omitted, e.g:

if (1 === 1) {
    console.log('Yes, 1 equals 1!'); // this line will be displayed on the screen
}

if ([1, 2].includes(1)) {
  console.log('Yes, this array contains the number 1'); // this line will be displayed on the screen
}

if ('') {
  console.log('an empty string is evaluated on false'); // this line will NOT be displayed on the screen
}

if ({} && []) {
  console.log('An empty array and an empty object evaluate themselves as true'); // this line will NOT be displayed on the screen
}

if (null || undefined) {
  console.log('null and undefined evaluate themselves on false'); // this line will NOT be displayed on the screen
}

if else and else¶

Each conditional statement if can have multiple code blocks defined, which will be executed when each of the previous conditions following the word if is evaluated as false. These blocks are else if. It is also possible to define a code block after the keyword else, which will be executed when none of the previous conditions in blocks of if or else if are not met, e.g:

const number = 5;

if (number % 5 === 4) {
  console.log('is your number 9?');
} else if (number % 5 === 3) {
  console.log('The remaiander is 3!');
} else if (number > 5) {
  console.log('I dont think its possible');
} else {
  console.log('All \'ifs \ were false so we landed here');
}

Switch¶

The switch instruction in the JavaScript is used to select the appropriate block to be executed, based on a certain value. Each possibility is printed with the keyword case and then, after a colon, the code that should be executed. A block of such a code can be terminated with the break instruction, which will exit the switch instruction. This statement also offers the possibility to define a block of default, which will be executed if the value does not match any of the case blocks defined.

Example:

const value = 5;

switch (value) {
    case 2: 
    case 4:
        console.log('I am definately even and lower than 5');
        break;
    case 1:
    case 3:
        console.log('I am odd and lower than 5');
        break;
    case 5:
        console.log('I am equal to 5');
        break;
    default:
        console.log('I am greater than 5');
}
// 'I am equal to 5' will be displayed

__NOTE_:_ The instructions of if else together with else work the same way as the switch instructions and can be used interchangeably. If you use the if else instructions, when the number of blocks of if else increases significantly, consider whether the switch instruction will not be clearer.