In programming, there may arise a situation where you have to run a block of code among more than one alternatives. For example, assigning grades A, B or C based on marks obtained by a student.

In such situations, you can use the JavaScript if...else statement to create a program that can make decisions.

In JavaScript, there are three forms of the if...else statement.

1. if statement
2. if…else statement
3. if…else if…else statement

JavaScript if Statement

The syntax of the if statement is:

if (condition) {
    // the body of if
}

The if statement evaluates the condition inside the parenthesis ().

1. If the condition is evaluated to true, the code inside the body of if is executed.
2. If the condition is evaluated to false, the code inside the body of if is skipped.

Note: The code inside { } is the body of the if statement.

Example 1: if Statement

// check if the number is positive

let number = prompt("Enter a number: ");

// check if number is greater than 0
if (number > 0) {
 // the body of the if statement
  console.log("The number is positive");
}

console.log("The if statement is easy");

Output 1

Enter a number: 2
The number is positive
The if statement is easy

Suppose the user entered 2. In this case, the condition number > 0 evaluates to true. And, the body of the if statement is executed.

Output 2

Enter a number: -1
The if statement is easy

Suppose the user entered -1. In this case, the condition number > 0 evaluates to false. Hence, the body of the if statement is skipped.

Since console.log("The if statement is easy"); is outside the body of the if statement, it is always executed.

Comparison and logical operators are used in conditions. So to learn more about comparison and logical operators, you can visit JavaScript Comparison and Logical Operators.

JavaScript if…else statement

An if statement can have an optional else clause. The syntax of the if...else statement is:

if (condition) {
    // block of code if condition is true
} else {
   // block of code if condition is false
}

The if..else statement evaluates the condition inside the parenthesis.

If the condition is evaluated to true,

1. the code inside the body of if is executed
2. the code inside the body of else is skipped from execution

If the condition is evaluated to false,

1. the code inside the body of else is executed
2. the code inside the body of if is skipped from execution

Example 2 : if…else Statement

// check is the number is positive or negative/zero

let number = prompt("Enter a number: ");

// check if number is greater than 0
if (number > 0) {
  console.log("The number is positive");
}
// if number is not greater than 0
else {
  console.log("The number is either a negative number or 0");
}

console.log("The if...else statement is easy");

Output 1

Enter a number: 2
The number is positive
The if...else statement is easy

Suppose the user entered 2. In this case, the condition number > 0 evaluates to true. Hence, the body of the if statement is executed and the body of the else statement is skipped.

Output 2

Enter a number: -1
The number is either a negative number or 0
The if...else statement is easy

Suppose the user entered -1. In this case, the condition number > 0 evaluates to false. Hence, the body of the else statement is executed and the body of the if statement is skipped.

JavaScript if…else if statement

The if...else statement is used to execute a block of code among two alternatives. However, if you need to make a choice between more than two alternatives, if...else if...else can be used.

The syntax of the if...else if...else statement is:

if (condition1) {
    // code block 1
} else if (condition2){
    // code block 2
} else {
    // code block 3
}

• If condition1 evaluates to true, the code block 1 is executed.
• If condition1 evaluates to false, then condition2 is evaluated.
  • If the condition2 is true, the code block 2 is executed.
  • If the condition2 is false, the code block 3 is executed.

Example 3: if…else if Statement

// check if the number if positive, negative or zero
let number = prompt("Enter a number: ");


// check if number is greater than 0
if (number > 0) {
    console.log("The number is positive");
}
// check if number is 0
else if (number == 0) {
  console.log("The number is 0");
}
// if number is neither greater than 0, nor zero
else {
    console.log("The number is negative");
}

console.log("The if...else if...else statement is easy");

Output

Enter a number: 0
The number is 0
The if...else if...else statement is easy

Suppose the user entered 0, then the first test condition number > 0 evaluates to false. Then, the second test condition number == 0 evaluates to true and its corresponding block is executed.

Nested if…else Statement

You can also use an if...else statement inside of an if...else statement. This is also known as nested if…else statement.

Example 4: Nested if…else Statement

// check if the number is positive, negative or zero
let number = prompt("Enter a number: ");

if (number >= 0) {
    if (number == 0) {
        console.log("You entered number 0");
    } else {
        console.log("You entered a positive number");
    }
} else {
    console.log("You entered a negative number");
}

Output

Enter a number: 0
You entered number 0

Suppose the user entered 0. In this case, the condition number > -1 evaluates to true. Then the second condition number == 0 is checked which evaluates to true and its corresponding body is executed.

Note: As you can see, nested if...else makes your logic complicated. If possible, you should always try to avoid nested if...else.

Body of if…else With Only One Statement

If the body of if...else has only one statement, you can omit { } in the program. For example, you can replace

let number = 2;
if (number > 0) {
    console.log("The number is positive.");
} else {
   console.log("The number is negative or zero.");
}

with

let number = 2;
if (number > 0)
    console.log("The number is positive.");
 else 
   console.log("The number is negative or zero.");

The output of both programs will be the same.

Output

The number is positive.

Note: Although it’s not necessary to use { } if the body of if...else has only one statement, using { } makes your code more readable.

More on Decision Making

In certain situations, a ternary operator can replace an if...else statement. To learn more, visit JavaScript Ternary Operator.

If you need to make a choice between more than one alternatives based on a given test condition, the switch statement can be used. To learn more, visit JavaScript switch.