In JavaScript, control structures are blocks of code that allow you to control the flow of your program. They enable you to make decisions based on certain conditions and to repeat a block of code multiple times.
There are several control structures in JavaScript, including if/else
statements, for
loops, and while
loops. In this article, we’ll go over the basics of each of these control structures and how to use them in your code.
If/Else Statements
An if/else
statement allows you to execute a block of code if a certain condition is true, or another block of code if the condition is false.
Here’s the syntax for an if/else
statement:
if (condition) {
// code to execute if condition is true
} else {
// code to execute if condition is false
}
Here’s an example of how to use an if/else
statement:
var age = 25;
if (age > 18) {
console.log("You are an adult.");
} else {
console.log("You are a minor.");
}
In this example, the if
block will be executed because the value of age
is greater than 18.
You can also use multiple else if
blocks to check for additional conditions:
var age = 25;
if (age < 18)
console.log("You are a minor.");
} else if (age < 21) {
console.log("You are a young adult.");
} else {
console.log("You are an adult.");
}
In this example, the else if
block will be executed because the value of age
is less than 21 but greater than or equal to 18.
For Loops
A for
loop is used to execute a block of code multiple times. It consists of three parts: an initializer, a condition, and an incrementer.
Here’s the syntax for a for
loop:
for (initializer; condition; incrementer) {
// code to execute
}
Here’s an example of how to use a for
loop:
for (var i = 0; i < 5; i++) {
console.log("Hello, world!");
}
In this example, the for
loop will execute the console.log
statement five times, with the value of i
starting at 0 and incrementing by 1 each time.
While Loops
A while
loop is similar to a for
loop, but it only has a condition. It will continue to execute a block of code as long as the condition is true.
Here’s the syntax for a while
loop:
while (condition) {
// code to execute
}
Here’s an example of how to use a while
loop:
var i = 0;
while (i < 5) {
console.log("Hello, world!");
i++;
}
In this example, the while
loop will execute the console.log
statement five times, with the value of i
starting at 0 and incrementing by 1 each time.
Conclusion
Control structures are an essential part of JavaScript programming. By understanding how to use if/else
statements, for
loops, and while
loops, you’ll have the skills you need to build complex and interactive web applications with JavaScript.
Exercises
To review these concepts, we will go through a series of exercises designed to test your understanding and apply what you have learned.
Use an if/else
statement to check whether a number is even or odd.
var num = 10;
if (num % 2 == 0) {
console.log("The number is even.");
} else {
console.log("The number is odd.");
}
Use a for
loop to print the numbers 1 to 10 to the console.
for (var i = 1; i <= 10; i++) {
console.log(i);
}
Use a while
loop to print the numbers 1 to 10 to the console.
var i = 1;
while (i <= 10) {
console.log(i);
i++;
}
Use a for
loop to print the numbers 10 to 1 to the console.
for (var i = 10; i > 0; i--) {
console.log(i);
}
Use a while
loop to print the numbers 10 to 1 to the console.
var i = 10;
while (i > 0) {
console.log(i);
i--;
}