In this blog post, we will see how can we use the if and else constructs in the Typescript program.

This is part of the Typescript Tutorial Series, whose index / TOC is available here → Typescript Tutorial Series.

If construct

if(`<expression>`) {
  //executable statement
}

In the above snippet, the executable statement gets executed, ONLY if the expression evaluates to true.

If-Else construct

An else clause is used to provide an alternate to the if statement, when the condition is not evaluates to true.

if(`<expression>`) {
  //executable statement 1
} else {
   //executable statement 2
}

In the above snippet, if the expression evaluates to true, the executable statement 1 gets executed. Otherwise, the executable statement 2 gets executed.

Example

Example - if statement

typescriptPractices > ls if-st*
if-statement.ts

typescriptPractices > cat if-statement.js
var age = 39;
if (age > 40) {
    console.log("Welcome Adult");
}
console.log("Your age is : " + age);

typescriptPractices > tsc if-statement.ts

typescriptPractices > node if-statement.js
Your age is : 39

You can see that the program did NOT print the greeting (Welcome statement) as the condition did not evaluate to true → 39 is not greater than 40. Instead it just printed the age at the end of the program which was unconditionally executed, meaning the statement was not bound to any condition.


// Make changes to the source code to change the value from 39 to 41
typescriptPractices > cat if-statement.js
var age = 41;
if (age > 40) {
    console.log("Welcome Adult");
}
console.log("Your age is : " + age);

typescriptPractices > tsc if-statement.ts  

typescriptPractices > node if-statement.js
Welcome Adult
Your age is : 41

typescriptPractices >

You can see that the program did print the greeting (Welcome statement) as the condition was evaluated to true → 41 is greater than 40.

It also printed the age at the end of the program which was unconditionally executed, meaning the statement was not bound to any condition.

Example - if-else statements

Now, we will see a sample program with the if-else constructs to have two different / alternate behaviors.

Source : if-else.ts

typescriptPractices > ls -l *if-e*
-rw-rw-rw-  1 raghs 0 174 2021-06-22 01:26 if-else.ts

typescriptPractices > cat if-else.ts
let myAge : number = 39;

if( myAge > 40 ) {
    console.log("Welcome Adult");
} else {
    console.log("Welcome Younster");
}

console.log("Your age is : " + myAge);

typescriptPractices > 

Transpile and Verify :

typescriptPractices > tsc if-else.ts

typescriptPractices > ls -l *if-el*
-rw-rw-rw-  1 raghs 0 163 2021-06-22 01:29 if-else.js
-rw-rw-rw-  1 raghs 0 174 2021-06-22 01:26 if-else.ts

typescriptPractices > 

Execute :

typescriptPractices > node if-else.js
Welcome Younster
Your age is : 39

You can see that the program printed the output in the else clause as the condition was not evaluated to true → Age 39 is NOT less than 40 and it has also printed the age as part of the last line which is an unconditional execution.

References

Cheers,
RM…
Raghavan alias Saravanan Muthu
22 Jun 2021 | Tue | 01:17:28 AM IST