TypeScript - Ifelse Statement

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In TypeScript, the if...else statement controls the program's execution flow based on the different conditions. If the condition evaluates to true, the if block of code is executed.

An if can be followed by an optional else block. The else block will execute if the Boolean expression tested by the if statement evaluates to false.

Syntax

The simple if...else statement in TypeScript is as follows

if(boolean_expression) {  
   // statement(s) will execute if the boolean expression is true
} else {
   // statement(s) will execute if the boolean expression is false  
}

Flowchart

The following flow chart shows how the if...else statement works.

The if block guards the conditional expression. The block associated with the if statement is executed if the Boolean expression evaluates to true.

The if block may be followed by an optional else statement. The instruction block associated with the else block is executed if the expression evaluates to false.

Examples

Let's understand the if...else statement in details with the help of some examples in TypeScript.

Example: Simple if...else

In the example below, the variable num is assigned the value 12. The condition (num % 2 == 0) checks if num is even. Since 12 divided by 2 has no remainder, the condition evaluates to true, and the block code following the if statement executes.

var num: number = 12; 
if (num % 2==0) { 
   console.log("Even"); 
} else {
   console.log("Odd"); 
}

On compiling, it will generate the following JavaScript code

var num = 12;
if (num % 2 == 0) {
   console.log("Even");
} else {
   console.log("Odd");
}

The above example prints whether the value in a variable is even or odd. The if block checks the divisibility of the value by 2 to determine the same. Here is the output of the above code

Even 

Example: Condition evaluates false

In the above example, if you assign the value 13 to the variable num, the condition becomes false because 13 divided by 2 leaves a remainder of 1. So the block code following the else statement executes.

var num: number = 13; 
if (num % 2==0) { 
   console.log("Even"); 
} else {
   console.log("Odd"); 
}

On compiling, it will generate the following JavaScript code.

var num = 13; 
if (num % 2==0) { 
   console.log("Even"); 
} else {
   console.log("Odd"); 
}

Here the condition (13 % 2 == 0) evaluates to false and the else block executes. The output is as follows

Odd

Example: else if statement

let grade: number = 85;
if (grade >= 90) {
  console.log("Excellent");
} else if (grade >= 80) {
  console.log("Great");
} else {
  console.log("Keep studying");
}

Here, the if statement first checks for the condition, grade >=90. If the condition evaluates to false, the else if statement checks for the condition, grade >= 80. Finally, else block executes only if conditions are false.

On compiling, it will generate the following JavaScript code.

let grade = 82;
if (grade >= 90) {
  console.log("Excellent");
} else if (grade >= 80) {
  console.log("Great");
} else {
  console.log("Keep studying");
}

Here, the condition of the else if statement evaluates to true, so it prints Great as output.

Great