A nested if statement in TypeScript is an if statement that is present inside the body of another if or else statement. The else…if ladder is a type of nested if statement. The nested if statement or else…if ladder is useful to test multiple conditions. Its syntax is given below −

Syntax

if (boolean_expression1) { 
   //statements if the expression1 evaluates to true 
} else if (boolean_expression2) { 
   //statements if the expression2 evaluates to true 
} else if (boolean_expression3) { 
   //statements if the expression3 evaluates to false 
} else {
   //statements if all three boolean expressions result to false
}

When using if…else…if and else statements, there are a few points to keep in mind.

• An if can have zero or one else”s and it must come after any else…if”s.

• An if can have zero to many else…if”s and they must come before the else.

• Once an else…if succeeds, none of the remaining else…if”s or else”s will be tested.

Example: else…if ladder

var num:number = 2 
if(num > 0) { 
   console.log(num+" is positive") 
} else if(num < 0) { 
   console.log(num+" is negative") 
} else { 
   console.log(num+" is neither positive nor negative") 
}

The snippet displays whether the value is positive, negative or zero.

On compiling, it will generate the following JavaScript code −

//Generated by typescript 1.8.10
var num = 2;
if (num > 0) {
   console.log(num + " is positive");
} else if (num < 0) {
   console.log(num + " is negative");
} else {
   console.log(num + " is neither positive nor negative");
}

Here is the output of the above code −

2 is positive