Summary - Conditionals

Combine if, switch and optional chaining to write a day-of-week app.

Day-of-week app

In this summary we will combine if, switch and optional chaining to build a day-of-week application. The app takes a day number and prints the day name; it does not throw on missing data.

The flow is: we read the day field of a config object safely with config?.day; we convert the day number to a name with switch; we catch unknown values with default. This way we handle missing data and multi-case scenarios together, as in a real app.

Exercise Print the day name

The config variable holds { day: 2 }. Declare a variable day and assign config?.day to it. Then write a switch (day) block: case 1 prints "Monday", case 2 prints "Tuesday" (each case ends with break), the default prints "Unknown". The output on a single line must be Tuesday.

Show example solution

const config = { day: 2 };
const day = config?.day;
switch (day) {
    case 1:
        console.log("Monday");
        break;
    case 2:
        console.log("Tuesday");
        break;
    default:
        console.log("Unknown");
}

Test yourself

Write the switch case

switch (day) {
____ 1:
        console.log("Monday");
        break;
    default:
        console.log("Unknown");
}

Show answer

switch (day) {
case 1:
        console.log("Monday");
        break;
    default:
        console.log("Unknown");
}

What does config?.day return when config is null?
```
const config = null;
config?.day
```
- undefined
- throws
- 0
- "Monday"
Show answer

undefined — ?. does not throw on null; it returns undefined.
Match each piece to its job

if

Tests one condition
switch

Multi-equality select
?

Ternary operator
?.

Null-safe access

Next lesson: The for loop

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