Explicit type annotations in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an explicit type annotation in TypeScript?

An explicit type annotation is when you directly specify the type of a variable, function parameter, or return value using a colon and the type name, like <code>let age: number;</code>. It helps the compiler know exactly what type to expect.

beginner

Why use explicit type annotations instead of relying on TypeScript's type inference?

Explicit type annotations make your code clearer and easier to understand. They also help catch mistakes early by ensuring variables have the exact type you want, especially when the inferred type might be too broad or unclear.

beginner

How do you add an explicit type annotation to a function parameter in TypeScript?

You add a colon and the type after the parameter name. For example: <code>function greet(name: string) { ... }</code> means the parameter <code>name</code> must be a string.

beginner

What happens if you assign a value of the wrong type to a variable with an explicit type annotation?

TypeScript will show an error during compilation because the value does not match the declared type. This helps prevent bugs by catching type mistakes early.

beginner

Show an example of explicit type annotation for a variable and a function return type.

Example:<br><code>let count: number = 5;</code><br>Here, <code>count</code> is explicitly a number.<br><br>Function with return type:<br><code>function getName(): string { return 'Alice'; }</code><br>This means the function must return a string.

What does this TypeScript code mean?<br>let price: number = 10;

A The variable price can only hold numbers.

B The variable price can hold any type.

C The variable price is a string.

D The variable price is undefined.

The explicit type annotation : number means price can only hold numbers.

How do you specify a function parameter must be a string?

A function greet(name string) {}

B function greet(name) {}

C function greet(string name) {}

D function greet(name: string) {}

You add a colon and the type after the parameter name: name: string.

What error will TypeScript show for this code?<br>let age: number = 'twenty';

A Error: Type 'string' is not assignable to type 'number'.

B No error, it's valid.

C Error: Missing type annotation.

D Error: Variable age is undefined.

Assigning a string to a variable declared as number causes a type error.

Why might you add explicit return type annotations to functions?

A To make the function run faster.

B To avoid writing function code.

C To help TypeScript check the function returns the correct type.

D To allow any return type.

Explicit return types help TypeScript verify the function returns what you expect.

Which is a correct explicit type annotation for a boolean variable?

A let isOpen = true: boolean;

B let isOpen: boolean = true;

C let isOpen: bool = true;

D let isOpen: Boolean = 'true';

The correct type is boolean (all lowercase), and the value must be true or false.

Explain what explicit type annotations are and why they are useful in TypeScript.

Think about how telling the computer exactly what type a variable should be helps.

Write a simple TypeScript function with explicit type annotations for parameters and return type.

Use a function that takes a string and returns a string.