This topic is designed to give you a better understanding of the TS type system (type programming), write your own type tools, or simply enjoy the challenge!

Implement a Pick

Implement the built-in Pick<T, K> for TS, but do not use it.

Select attribute K from type T to construct a new type.

Such as:

interface Todo {
  title: string
  description: string
  completed: boolean
}

type TodoPreview = MyPick<Todo, 'title' | 'completed'>

const todo: TodoPreview = {
    title: 'Clean room',
    completed: false,
}
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The answer:

Realize the Readonly

Don’t use the built-in Readonly, implement one yourself.

This Readonly takes a generic parameter and returns exactly the same type, except that all properties are embellished by Readonly.

That is, you can no longer assign values to attributes of the object.

Such as:

interface Todo {
  title: string
  description: string
}

const todo: MyReadonly<Todo> = {
  title: "Hey",
  description: "foobar"
}

todo.title = "Hello" // Error: cannot reassign a readonly property
todo.description = "barFoo" // Error: cannot reassign a readonly property
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The answer:

Tuples are converted to objects

Pass in a tuple type and convert the tuple type to an object type whose keys/values are iterated from the tuple.

Such as:

const tuple = ['tesla', 'model 3', 'model X', 'model Y'] as const

const result: TupleToObject<typeof tuple> // expected { tesla: 'tesla', 'model 3': 'model 3', 'model X': 'model X', 'model Y': 'model Y'}
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The answer:

The first element

Implement a generic First that takes an array T and returns the type of its First element.

Such as:

type arr1 = ['a', 'b', 'c']
type arr2 = [3, 2, 1]

type head1 = First<arr1> // expected to be 'a'
type head2 = First<arr2> // expected to be 3
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The answer:

Gets the length of the element

Create a generic Length that takes a readonly array and returns the Length of the array.

Such as:

type tesla = ['tesla', 'model 3', 'model X', 'model Y']
type spaceX = ['FALCON 9', 'FALCON HEAVY', 'DRAGON', 'STARSHIP', 'HUMAN SPACEFLIGHT']

type teslaLength = Length<tesla> // expected 4
type spaceXLength = Length<spaceX> // expected 5
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The answer:

To realize the Exclude

Implement the built-in Exclude <T, U>

Exclude from T those types that can be assigned to U

The answer:

Awaited

If we have a Promise object, the Promise object will return a type. In TS, we use the T in the Promise to describe the type returned by this Promise. You implement a type that can be retrieved.

For example: Promise, return type ExampleType.

The answer:

If

Implement an If util that takes a C generic argument and returns type T If true and type F If false

Such as:

  type A = If<true, 'a', 'b'>  // expected to be 'a'
  type B = If<false, 'a', 'b'> // expected to be 'b'
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Concat

Implements a Concat version of the array type

Such as:

 type Result = Concat<[1], [2]> // expected to be [1, 2]
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The answer:

Includes

❝

Implement the JavaScript array.includes function in the type system. A type takes two arguments. The output should be a Boolean value of true or false.

❞

Such as:

type isPillarMen = Includes<['Kars', 'Esidisi', 'Wamuu', 'Santana'], 'Dio'> // expected to be `false`
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This problem is difficult, anyway I am copy!!

Push

Implement the array. push version

Such as:

  type Result = Push<[1, 2], '3'> // [1, 2, '3']
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The answer:

Unshift

Implement the type version of array. unshift

Such as:

type Result = Unshift<[1, 2], 0> // [0, 1, 2,]
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The answer:

The last

I’ve done all of these before, so please leave a comment in the comments section

reference

  • Github.com/type-challe…
  • www.typescriptlang.org/docs/handbo…