Handbook: Basic Types

Question 1

Give an example of a TS boolean assignment statement.

Answer 1

let isDone: boolean = false;

Question 2

In TS, all numbers are ____

Answer 2

… floating point values.

Question 3

Give 4 examples of number assignments, each using a different literal type.

Answer 3

let decimalNum: number = 7;

let hexNum: number = 0xf00d;

let binaryNum: number = 0b1010;

let octalNum: number = 0o744;

Question 4

Give an example of a string assignment.

Answer 4

let description: string = 'A great text for all ages';

Question 5

What are the two ways to define an array of number values [1, 2, 3]?

Answer 5

let list: number[] = [1, 2, 3]

let list: Array<number> = [1, 2, 3]</number>

Question 6

What are tuples and give an example?

Answer 6

Tuples allow you to define a fixed number of elements in an array, where the values may be of varrying types.

let greeting: [string, number] = ["Hello", 24601]

Question 7

When accessing an element outside of the known indices in a tuple, a ___ type is used instead

Answer 7

union

Question 8

What do enums do?

Answer 8

They give a more friendly name to sets of numeric values.

Question 9

Describe enum numbering

Answer 9

• Enums begin numbering their members at 0
• You can manually set the values of members
• If you don’t set the first element, it will always start at 0, even if you set a later one to 0

Question 10

Create an enum of the 3 primary colors.

Access the numeric value of one of them.

Also access the name value of the second member and what does it print?

Answer 10

enum Color {Red = 2, Green, Blue}

Color.Green

Color[3] - prints Green

Question 11

What does the any type allow?

Answer 11

It allows us to opt-out of type checking during compile-time checks for values that may be coming from dynamic content or 3rd party libraries, or we might for some other reason, not know its type.

Question 12

Why doesn’t the following work?

`let aNum: Object = 4

aNum.toFixed()`

Answer 12

toFixed is a method from Number.prototype.

That method does not exist on the base Object JavaScript class, which is what we defined our variable to be.

Question 13

Give a handy trick that you can use any for.

Answer 13

If you know part of the type but not all of it. Such as, you know it’s an array but you don’t know the types that will fill it up.

let userData: any[] = ['hey', 24601]

let userData: Array<any> = ['hey', 24601]</any>

Question 14

What is void for, and when do you commonly see it?

What’s one more notable thing about in in relation to variables?

Answer 14

void is the absense of having any type at all, commonly seen as the return type of functions that do not return a value.

`function warnUser(): void {

console.log(‘Warning’)

}`

It’s not useful to declare variables of type void because only undefined and null can be then assigned to the variable.

Question 15

Describe undefined and null.

Answer 15

• They each of their own type, respectively undefined and null.
  
  • let u: undefined = undefined
• Both types are, by default, subtypes of all other types. This means that you can assign undefined to a number type.
  
  • Using the --strictNullCheck flag, undefined and null can only be assigned to void and their respective types
• If wanted a string to optionally be an actual string literal or null, better to use an explicity union type:
  
  • let userNum: string | null

Question 16

What does the type never represent?

Answer 16

• Represents the types of values that never occur.
• For example, never is the return type of a function that always throws an error or one that never completes
  
  • A function can have an inferred type of never if it’s found to never be able to complete

Question 17

Give an example of a function with a never type that does not error out.

Answer 17

`function keepDoing(): never {

while(true) {}

}`

Question 18

Describe the object type

Answer 18

• object is a type that describes the non-primitive type.
  
  • anything that is not number, string, boolean, symbol, null, or undefined

Question 19

What is a type assertion?

Answer 19

A type assertian is a well to tell the compiler that you know more about a variable’s type than it does. This usually happens when you know that it’s more specific than it’s currently described as.

Question 20

Give two ways to represent a type assertion?

What’s special about one of them?

Answer 20

`let someValue: any = “I’m a cool string”

let lengthVal: number = (<string>someValue).length`</string>

`let someValue: any = “I’m a cool string again”

let lengthVal: number = (someValue as string).length`

This first method is not permitted in JSX. You must use the as syntax in JSX.