Array type annotation syntax in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the basic syntax to annotate an array of numbers in TypeScript?

You can annotate an array of numbers using <code>number[]</code>. For example: <code>let nums: number[] = [1, 2, 3];</code>

beginner

How do you annotate an array of strings using the generic Array type syntax?

Use the generic form <code>Array&lt;string&gt;</code>. For example: <code>let names: Array&lt;string&gt; = ['Alice', 'Bob'];</code>

intermediate

Can you mix types in an array annotation? How?

Yes, by using a union type inside the array annotation. For example: <code>let mixed: (number | string)[] = [1, 'two', 3];</code>

beginner

What is the difference between number[] and Array<number>?

Both mean the same: an array of numbers. number[] is shorthand, and Array<number> is the generic form. Use whichever you find clearer.

intermediate

How do you annotate a two-dimensional array of booleans?

You can write <code>boolean[][]</code> or <code>Array&lt;Array&lt;boolean&gt;&gt;</code>. For example: <code>let grid: boolean[][] = [[true, false], [false, true]];</code>

Which of these is a correct TypeScript annotation for an array of strings?

A Array<string>

B Both B and C

C string[]

D string()

Both string[] and Array<string> are valid ways to annotate an array of strings.

How would you annotate an array that can hold numbers or strings?

A (number | string)[]

B Array<number | string>

C Both A and C

D number[] | string[]

Both (number | string)[] and Array<number | string> correctly annotate an array holding numbers or strings.

What does boolean[][] represent?

A An array of booleans

B A boolean value

C An array of arrays of numbers

D A two-dimensional array of booleans

boolean[][] means an array where each element is an array of booleans.

Which syntax is NOT valid for annotating an array of numbers?

A Array<number[]>

B Array<number>

C number[]

D Both A and B

Array<number[]> means an array of arrays of numbers, not a simple array of numbers.

What is the shorthand syntax for Array<string>?

A string[]

B string()

C Array<string[]>

D string{}

string[] is the shorthand for Array<string>.

Explain how to annotate an array of mixed types in TypeScript.

Think about how to say 'this array can hold more than one type'.

Describe the difference and similarity between number[] and Array.

One is shorter, the other uses angled brackets.