Conditional statements¶

Conditional statements are language built-in mechanisms that execute the selected code depending on whether the value of the expression is true (true) or false (false). JavaScript offers the following options:

• if
• if - else
• else if
• switch

if¶

These instructions have a similar syntax to what you already know from Java. Ie. after the keyword if in parentheses we give the appropriate condition. If it evaluates to true then the code from the inner block (i.e. inside the {and } braces) will be executed, otherwise it is skipped, e.g .:

if (1 === 1) {
    console.log('Tak 1 jest rowne 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 evaluates to false'); // this line will NOT be displayed on the screen
}

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

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

if else and else¶

Each if conditional statement can define multiple blocks of code that will be executed when any of the preceding conditions following the word if evaluates to false. These blocks are else if. It is also possible to define a block of code after the keyword else, which will be executed in case none of the previous conditions in if orelse if blocks will be 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 statement¶

The switch statement in JavaScript performs the same function and has a very similar syntax as in [Java] (../../java_basics/control_statements.md # switch-statement).It plays the role of selecting the appropriate block to be executed based on the specified value. Each possibility is printed using the case keyword, followed by the code that should be executed after a colon. We can end a block of this code with a break statement, which will exit theswitch statement. This statement also offers the option of defining a default block, which will be executed in case the value does not match any of the definedcase blocks.

For example:

const value = 5;

switch (value) {
    case 2:
    case 4:
        console.log('I am definitely 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');
}
// will be displayed: I am equal to 5

NOTE: The if else statements, together withelse, work just like the switch statement, and we can use them interchangeably. If we use if else statements when the number ofif else blocks increases significantly, consider whether the switch statement will be clearer.