Type annotation on variables in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is type annotation on variables in TypeScript?

Type annotation is a way to tell TypeScript what type a variable should hold, like number, string, or boolean. It helps catch mistakes before running the code.

beginner

How do you add a type annotation to a variable in TypeScript?

You write a colon (:) after the variable name, then the type. For example: <code>let age: number;</code> means age can only hold numbers.

beginner

What happens if you assign a wrong type to a variable with type annotation?

TypeScript will show an error before running the program. For example, if <code>let name: string;</code> and you try <code>name = 5;</code>, it will warn you.

beginner

Can you give an example of type annotation with a variable holding a boolean?

Yes! For example: <code>let isOpen: boolean = true;</code> means the variable <code>isOpen</code> can only be true or false.

beginner

Why is type annotation useful when working with variables?

It helps you and the computer understand what kind of data the variable should have. This prevents bugs and makes your code easier to read and maintain.

How do you write a type annotation for a variable named score that holds numbers?

A let score = number;

B let score: number;

C let score: 'number';

D let score number;

The correct syntax is let score: number; where the colon (:) is followed by the type.

What will TypeScript do if you assign a string to a variable annotated as a number?

A Show an error before running the code

B Automatically convert the string to number

C Ignore the type and run anyway

D Delete the variable

TypeScript checks types before running and will show an error if types don't match.

Which of these is a valid type annotation for a boolean variable?

A let isReady: bool;

B let isReady: yesno;

C let isReady: Boolean;

D let isReady: boolean;

The correct type for boolean values in TypeScript is boolean (all lowercase).

What does this code mean? let name: string = 'Alice';

A Variable <code>name</code> must hold text and starts with 'Alice'

B Variable <code>name</code> can hold any type

C Variable <code>name</code> holds a number

D Variable <code>name</code> is undefined

The variable name is declared to hold text (string) and is initialized with 'Alice'.

Why might you want to use type annotations on variables?

A To make variables change type automatically

B To make the program run faster

C To catch mistakes early and make code clearer

D To hide the variable's value

Type annotations help catch errors early and improve code readability.

Explain what type annotation on variables means and how to write it in TypeScript.

Think about how you tell TypeScript what kind of data a variable should hold.

Describe why using type annotations on variables is helpful when writing code.

Consider how knowing the type of data helps both you and the computer.

Practice

1. What does the type annotation let age: number; tell us about the variable age?

easy

A. It means age can hold any type of value.

B. It means age can only hold numbers.

C. It means age is a string variable.

D. It means age is a function.

Solution

1. Step 1: Understand type annotation syntax

   The colon : after age followed by number means age must hold a number value.

2. Step 2: Interpret the meaning of number

   The type number restricts the variable to only numeric values, no strings or other types allowed.

3. Final Answer:

   It means age can only hold numbers. -> Option B

4. Quick Check:

   Type annotation : number means only numbers [OK]

Hint: Colon after variable name means type follows [OK]

Common Mistakes:

• Thinking type annotation changes the value
• Confusing type annotation with assignment
• Assuming variable can hold any type without error

2. Which of the following is the correct way to add a type annotation for a string variable named name in TypeScript?

easy

A. let name string;

B. let name = string;

C. let name: string;

D. let name: "string";

Solution

1. Step 1: Recall correct syntax for type annotation

   Type annotation uses a colon : followed by the type name without quotes.

2. Step 2: Check each option

   let name: string; uses let name: string; which is correct syntax. Options B and C miss the colon or use wrong syntax. let name: "string"; uses quotes around string, which is incorrect.

3. Final Answer:

   let name: string; -> Option C

4. Quick Check:

   Use colon and type name without quotes [OK]

Hint: Use colon then type name without quotes [OK]

Common Mistakes:

• Omitting the colon between variable and type
• Putting type in quotes like a string value
• Using equals sign instead of colon for type

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

let count: number = 5;
count = 10;
console.log(count);

medium

A. 10

B. Error: Type mismatch

C. undefined

D. 5

Solution

1. Step 1: Analyze variable declaration and assignment

   count is declared as a number and initialized to 5, then reassigned to 10, which is valid.

2. Step 2: Understand console.log output

   The last assigned value is 10, so console.log(count) prints 10.

3. Final Answer:

   10 -> Option A

4. Quick Check:

   Variable holds last assigned number value [OK]

Hint: Variable prints last assigned value [OK]

Common Mistakes:

• Thinking initial value 5 is printed
• Expecting error on reassignment with same type
• Confusing output with undefined

4. Identify the error in this TypeScript code:

let isActive: boolean = "true";

medium

A. No error, code is correct.

B. Missing semicolon at the end.

C. Variable name cannot be isActive.

D. Type mismatch: string assigned to boolean.

Solution

1. Step 1: Check type annotation and assigned value

   The variable isActive is typed as boolean but assigned a string value "true".

2. Step 2: Understand TypeScript type checking

   TypeScript does not allow assigning a string to a boolean variable, causing a type mismatch error.

3. Final Answer:

   Type mismatch: string assigned to boolean. -> Option D

4. Quick Check:

   Type annotation enforces matching value types [OK]

Hint: Check assigned value matches annotated type [OK]

Common Mistakes:

• Ignoring quotes around boolean values
• Assuming string "true" is same as boolean true
• Thinking semicolon or variable name causes error

5. You want to declare a variable data that can hold either a number or a string. Which is the correct TypeScript annotation?

hard

A. let data: number | string;

B. let data: number & string;

C. let data: number, string;

D. let data: (number string);

Solution

1. Step 1: Understand union types in TypeScript

   The pipe symbol | is used to allow a variable to hold one of multiple types.

2. Step 2: Evaluate each option

   let data: number | string; uses number | string which means data can be either number or string. let data: number & string; uses & which means intersection (both types at once), invalid here. Options C and D have incorrect syntax.

3. Final Answer:

   let data: number | string; -> Option A

4. Quick Check:

   Use | for multiple allowed types [OK]

Hint: Use pipe | to allow multiple types [OK]

Common Mistakes:

• Using & instead of | for multiple types
• Separating types with comma or space
• Putting types inside parentheses without operator