Declaring global variables in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of accessing a global variable inside a function

What is the output of this TypeScript code?

Typescript

let count = 10;
function showCount() {
  console.log(count);
}
showCount();

A 10

B undefined

C ReferenceError

D null

💡 Hint

Global variables are accessible inside functions unless shadowed.

The variable count is declared globally and accessible inside showCount. So it prints 10.

❓ Predict Output

intermediate

Effect of redeclaring a global variable inside a function

What will this code print?

Typescript

let message = "Hello";
function greet() {
  let message = "Hi";
  console.log(message);
}
greet();
console.log(message);

A

Hi
Hi

B

Hello
Hello

C

Hi
Hello

D ReferenceError

💡 Hint

Local variables with the same name shadow global ones inside functions.

The function prints the local message "Hi". Outside, the global message remains "Hello".

🔧 Debug

advanced

Why does this code cause an error?

What error does this TypeScript code produce?

Typescript

function update() {
  count = 5;
  let count = 10;
}
update();

A No error, runs fine

B ReferenceError: Cannot access 'count' before initialization

C TypeError: count is not a function

D SyntaxError: Unexpected token

💡 Hint

Variables declared with let are not hoisted like var.

The code tries to assign to count before it is declared with let, causing a ReferenceError.

❓ Predict Output

advanced

Global variable modification inside a function

What is the output of this code?

Typescript

let total = 0;
function add(value: number) {
  total += value;
}
add(5);
add(3);
console.log(total);

A NaN

B 0

C ReferenceError

D 8

💡 Hint

Functions can change global variables if they don't redeclare them.

The function adds values to the global total. After two calls, total is 8.

🧠 Conceptual

expert

Understanding global variable declaration in TypeScript modules

In a TypeScript file treated as a module, which statement correctly declares a global variable accessible across multiple files?

A Declare the variable with <code>declare global { var myVar: number; }</code> inside a <code>*.d.ts</code> file and assign it in one module.

B Declare the variable inside a function with <code>globalThis.myVar = 10;</code> without any declaration.

C Use <code>var myVar = 10;</code> inside a function to make it global.

D Use <code>let myVar = 10;</code> at the top of any module file.

💡 Hint

Global variables shared across modules require declaration merging or global augmentation.

Option A correctly uses declare global in a declaration file to extend the global scope. Other options either create local variables or do not properly declare globals.