Declaring global variables in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a global variable in TypeScript?

A global variable is a variable declared outside any function or block, accessible from anywhere in the code.

beginner

How do you declare a global variable in TypeScript?

You declare a global variable by defining it outside any function or block, usually at the top of your file or in a separate global declaration file.

beginner

What keyword is used to declare a variable that can be reassigned globally?

The let keyword is used to declare a variable that can be reassigned. If declared globally, it is accessible throughout the program.

intermediate

What is the difference between var and let when declaring global variables?

var declares a global variable that attaches to the global object (like window in browsers), while let declares a block-scoped variable that does not attach to the global object.

intermediate

How can you declare a global variable type in TypeScript for use across multiple files?

You can declare global variable types by creating a .d.ts file with a declare global block, then defining the variable type inside it.

Where should you declare a global variable in TypeScript?

A Outside any function or block

B Inside a function only

C Inside a class only

D Inside a block only

Global variables are declared outside any function or block so they can be accessed anywhere.

Which keyword declares a block-scoped global variable in TypeScript?

A var

B function

C let

D const

let declares a block-scoped variable that can be global if declared outside blocks.

What happens if you declare a global variable with var in a browser environment?

A It is only accessible inside functions

B It becomes a property of the global <code>window</code> object

C It is block-scoped

D It causes a syntax error

var global variables attach to the window object in browsers.

How do you declare a global variable type in TypeScript for multiple files?

A Using <code>const</code> inside a block

B Using <code>var</code> inside a function

C Using <code>let</code> inside a class

D Using <code>declare global</code> in a <code>.d.ts</code> file

Global types are declared with declare global in declaration files.

Which keyword should you avoid for global variables due to hoisting and global object pollution?

A var

B let

C const

D function

var can cause hoisting issues and pollutes the global object, so it's best to avoid it for globals.

Explain how to declare a global variable in TypeScript and the difference between using var and let.

Think about where the variable lives and how it behaves in the global scope.

Describe how to declare global variable types in TypeScript for use across multiple files.

This helps TypeScript understand global variables in big projects.

Practice

1. What is a global variable in TypeScript?

easy

A. A variable accessible anywhere in the program

B. A variable only accessible inside a function

C. A variable that cannot be changed

D. A variable declared inside a class only

Solution

1. Step 1: Understand variable scope

   Global variables are those that can be accessed from any part of the program, not limited to a function or block.

2. Step 2: Compare options

   A variable accessible anywhere in the program correctly describes a global variable. Options B, C, and D describe other types or restrictions that do not match global variables.

3. Final Answer:

   A variable accessible anywhere in the program -> Option A

4. Quick Check:

   Global variable = accessible everywhere [OK]

Hint: Globals are variables you can use anywhere in your code [OK]

Common Mistakes:

• Confusing global with local variables
• Thinking globals are immutable
• Assuming globals are only inside classes

2. Which of the following is the correct way to declare a global variable in TypeScript outside any function?

easy

A. let globalVar = 10;

B. var globalVar = 10;

C. const globalVar: number;

D. function globalVar() {}

Solution

1. Step 1: Identify global declaration syntax

   Using var outside any function declares a global variable in TypeScript.

2. Step 2: Analyze options

   var globalVar = 10; uses var correctly. let globalVar = 10; uses let which is block-scoped, not global. const globalVar: number; declares a constant without initialization, causing error. function globalVar() {} declares a function, not a variable.

3. Final Answer:

   var globalVar = 10; -> Option B

4. Quick Check:

   Use var outside functions for globals [OK]

Hint: Use var outside functions to create globals [OK]

Common Mistakes:

• Using let or const for globals outside modules
• Declaring variables without initialization
• Confusing functions with variables

3. What will be the output of this TypeScript code?

var count = 5;
function increment() {
  count++;
}
increment();
console.log(count);

medium

A. undefined

B. 5

C. 6

D. Error

Solution

1. Step 1: Understand variable scope and mutation

   The variable count is declared globally with value 5. The function increment increases it by 1.

2. Step 2: Trace code execution

   Calling increment() changes count from 5 to 6. Then console.log(count) prints 6.

3. Final Answer:

   6 -> Option C

4. Quick Check:

   Global var incremented once = 6 [OK]

Hint: Global var changes inside functions affect output [OK]

Common Mistakes:

• Assuming count stays 5 after increment
• Thinking count is local inside function
• Expecting undefined due to scope confusion

4. Identify the error in this TypeScript code that tries to declare a global variable inside a module:

export {};
declare global {
  var message: string;
}
message = "Hello";
console.log(message);

medium

A. Missing initialization of message inside declare global

B. declare global block must be inside a module file

C. message must be declared with let or const

D. Cannot assign to message outside declare global block

Solution

1. Step 1: Understand declare global usage

   The declare global block is used to add types to global variables but does not create or assign them.

2. Step 2: Identify assignment error

   Assigning message = "Hello"; outside the block causes error because message is only declared, not defined. You must define it separately.

3. Final Answer:

   Cannot assign to message outside declare global block -> Option D

4. Quick Check:

   declare global declares types, assignment needs separate definition [OK]

Hint: declare global only declares, assign globals separately [OK]

Common Mistakes:

• Trying to assign inside declare global block
• Confusing declaration with definition
• Using let or const inside declare global

5. You want to add a global variable appName of type string in a TypeScript module and assign it the value "MyApp" so it can be used anywhere. Which is the correct way?

hard

A. declare global { var appName: string; }
globalThis.appName = "MyApp";

B. declare global { var appName: string; }
appName = "MyApp";

C. export var appName = "MyApp";

D. let appName: string = "MyApp";

Solution

1. Step 1: Declare global variable type inside module

   Use declare global block to declare the variable type for global scope.

2. Step 2: Assign value to global variable correctly

   Assign the value to globalThis.appName to define the global variable at runtime.

3. Step 3: Analyze options

   declare global { var appName: string; } globalThis.appName = "MyApp"; correctly declares and assigns the global variable. declare global { var appName: string; } appName = "MyApp"; misses assigning to globalThis. export var appName = "MyApp"; exports a module variable, not global. let appName: string = "MyApp"; declares a local variable.

4. Final Answer:

   declare global { var appName: string; } globalThis.appName = "MyApp"; -> Option A

5. Quick Check:

   declare global + globalThis assignment = global var [OK]

Hint: Use declare global plus globalThis to define globals in modules [OK]

Common Mistakes:

• Assigning globals without globalThis
• Using export instead of global declaration
• Declaring local variables instead of globals