Posted At: Jul 18, 2024 - 246 Views
Conditional Branching: if
and ?
Sometimes, we need to perform different actions based on different conditions. To do that, we can use the if
statement and the conditional operator ?
, also known as the "question mark" operator.
The if
Statement
The if(...)
statement checks a condition in parentheses and, if the result is true, runs a block of code.
For example:
// JavaScript
let favoriteFruit = prompt('What is your favorite fruit?', '');
if (favoriteFruit === 'apple') {
alert('Apples are delicious!');
} else if (favoriteFruit === 'banana') {
alert('Bananas are great for a quick snack!');
} else {
alert(`Wow, ${favoriteFruit} is a unique choice!`);
}
// TypeScript
let favoriteFruit: string | null = prompt('What is your favorite fruit?', '');
if (favoriteFruit === 'apple') {
alert('Apples are delicious!');
} else if (favoriteFruit === 'banana') {
alert('Bananas are great for a quick snack!');
} else {
alert(`Wow, ${favoriteFruit} is a unique choice!`);
}
In this example, the condition checks the value of favoriteFruit
. Depending on the value, it displays a different message.
When we need to run more than one statement, we use curly braces:
// JavaScript
if (favoriteFruit === 'apple') {
alert('Apples are delicious!');
alert("They're also great in pies!");
}
// TypeScript
if (favoriteFruit === 'apple') {
alert('Apples are delicious!');
alert("They're also great in pies!");
}
Always using curly braces {}
helps with readability, even if there's only one statement.
Boolean Conversion
The if (...)
statement evaluates the expression in its parentheses and converts the result to a boolean.
Conversion rules:
- Falsy values:
0
,""
(empty string),null
,undefined
, andNaN
- Truthy values: All other values
Examples:
// JavaScript
if (0) {
// This code will not run
}
if (1) {
// This code will run
}
// TypeScript
if (0) {
// This code will not run
}
if (1) {
// This code will run
}
We can also pass a pre-evaluated boolean value to if
:
// JavaScript
let age = 25;
let isAdult = (age >= 18);
if (isAdult) {
alert('You are an adult!');
}
// TypeScript
let age: number = 25;
let isAdult: boolean = (age >= 18);
if (isAdult) {
alert('You are an adult!');
}
The else
Clause
The if
statement can have an optional else
block, which runs when the condition is false.
// JavaScript
let weather = prompt('Is it raining?', '');
if (weather === 'yes') {
alert('Take an umbrella!');
} else {
alert('Enjoy the sunshine!');
}
// TypeScript
let weather: string | null = prompt('Is it raining?', '');
if (weather === 'yes') {
alert('Take an umbrella!');
} else {
alert('Enjoy the sunshine!');
}
Several Conditions: else if
The else if
clause lets us check multiple conditions.
// JavaScript
let time = prompt('What time is it (0-23)?', '');
if (time < 12) {
alert('Good morning!');
} else if (time < 18) {
alert('Good afternoon!');
} else if (time < 22) {
alert('Good evening!');
} else {
alert('Good night!');
}
// TypeScript
let time: string | null = prompt('What time is it (0-23)?', '');
if (time !== null) {
let numTime: number = parseInt(time);
if (numTime < 12) {
alert('Good morning!');
} else if (numTime < 18) {
alert('Good afternoon!');
} else if (numTime < 22) {
alert('Good evening!');
} else {
alert('Good night!');
}
}
Conditional Operator ?
The conditional (ternary) operator allows us to assign a value based on a condition.
// JavaScript
let temperature = prompt('What is the temperature?', '');
let isCold = temperature < 15 ? true : false;
alert(isCold ? 'Brrr! It’s cold!' : 'Nice weather!');
// TypeScript
let temperature: string | null = prompt('What is the temperature?', '');
let isCold: boolean = temperature !== null && parseInt(temperature) < 15 ? true : false;
alert(isCold ? 'Brrr! It’s cold!' : 'Nice weather!');
Multiple ?
We can use multiple conditional operators to return a value based on several conditions.
// JavaScript
let age = prompt('How old are you?', '');
let message = (age < 5) ? 'Awww, so young!' :
(age < 13) ? 'Hey there, kiddo!' :
(age < 20) ? 'What’s up, teenager?' :
(age < 30) ? 'Hello, young adult!' :
(age < 65) ? 'Greetings, adult!' :
'Respect, senior citizen!';
alert(message);
// TypeScript
let age: string | null = prompt('How old are you?', '');
let message: string = age !== null ? (
parseInt(age) < 5 ? 'Awww, so young!' :
parseInt(age) < 13 ? 'Hey there, kiddo!' :
parseInt(age) < 20 ? 'What’s up, teenager?' :
parseInt(age) < 30 ? 'Hello, young adult!' :
parseInt(age) < 65 ? 'Greetings, adult!' :
'Respect, senior citizen!'
) : 'Invalid age';
alert(message);
Non-traditional Use of ?
The question mark ?
can replace if
, but it is generally less readable.
// JavaScript
let answer = prompt('Do you like JavaScript?', '');
answer === 'yes' ? alert('Great!') : alert('Oh no, why not?');
// JavaScript
let answer = prompt('Do you like JavaScript?', '');
answer === 'yes' ? alert('Great!') : alert('Oh no, why not?');
For better readability, use if
instead:
// JavaScript
let answer = prompt('Do you like JavaScript?', '');
if (answer === 'yes') {
alert('Great!');
} else {
alert('Oh no, why not?');
}
// JavaScript
let answer = prompt('Do you like JavaScript?', '');
if (answer === 'yes') {
alert('Great!');
} else {
alert('Oh no, why not?');
}
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