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.