Double Assertions

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Professional TypeScript Masterclass Lessons

3.Primitive Types

4.Instance Types

5.Arrays And Tuples

7.const declarations

8.Function Types

9.Structural Typing

10.Classes in TypeScript

11.Target Compiler Option

13.Special Types any And unknown

14.JavaScript to TypeScript

15.Frontend Projects

16.Type Assertions

17.Type Casting

19.Type Declarations

20.Creating NPM packages

22.Running in NodeJS

23.Lexical this

24.readonly Modifier

26.Literal Types

27.Type Narrowing

28.Discriminated Unions

29.Class Parameter Properties

30.Strict Compiler Option

31.null vs undefined

32.Intersection Types

33.Optional Modifier

34.Non Null Assertion Operator

36.Interface Declaration Merging

37.Types vs Interfaces

39.implements Keyword

40.Definite Assignment Assertion

41.User Defined Type Guards

42.Assertion Functions

43.Function Overloading

44.Call Signatures

45.Abstract Classes

46.Index Signatures

47.Readonly Arrays and Tuples

48.Double Assertions

49.const Assertions

50.this Parameter

51.Generic Constraints

52.typeof Type Operator

53.Lookup Types

54.keyof Type Operator

55.Conditional Types

56.Contitional Types with Unions and never

57.infer Keyword and `ReturnType<T>`

58.Mapped Types

59.Mapped Type Modifiers

60.Template Literal Type

64.Record<K, T>

65.Project References

66.undefined vs. optional

67.satisfies Operator

68.PropertyKey Type

71.String Manipulation Types

72.Mapped Types as Clauses

73.Union vs Intersection Mental Model

74.Enums are Bad

Double Assertions

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Single Assertion

Single assertions are allowed if the types are compatible in one direction. e.g.:

type Point2D = { x: number, y: number };
type Point3D = { x: number, y: number, z: number };

let point2: Point2D = { x: 0, y: 0 };
let point3: Point3D = { x: 10, y: 10, z: 10 };

point3 = point2; // Error
point3 = point2 as Point3D; // Ok: I trust you

If they are not compatibility in either direction then TypeScript will not allow the assertion e.g.:

type Point2D = { x: number, y: number };
type Person = { name: string, email: string };

let point2: Point2D = { x: 0, y: 0 };
let person: Person = { name: 'john', email: 'john@example.com' };

point2 = person; // Error
point2 = person as Point2D; // Error: I don't trust you enough

Double Assertion

We can assert to unknown as a middle step. Since unknown is compatible with all types, TypeScript allow the next assertion e.g.:

type Point2D = { x: number, y: number };
type Person = { name: string, email: string };

let point2: Point2D = { x: 0, y: 0 };
let person: Person = { name: 'john', email: 'john@example.com' };

point2 = person; // Error
point2 = person as unknown as Point2D; // Ok: I trust you

Further

Some people use as any (instead of as unknown). They are both equivalent, but prefer using unknown as it is common to search a codebase for any as a sign of code smell 💩.

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Transcript

00:00

Here we have a type 0.2 D with numbers X and Y of type number and a type 0.3 D with numbers X, Y and Z of type number. And then we have a type person with a name in an email. Then we create variables of each of these three types, so we can do some type comparison between them. Remember, the types in TypeScript are structural. This means that we can assign a 0.3 D to a 0.2 D without any errors because a 0.3 D does have all the members that are required by a 0.2 D, which is X and Y.

00:33

However, we cannot assign a 0.2 D to a 0.3 D because a 0.3 D requires a member Z, and this variable 0.2 simply does not have that. However, we can force this assignment to happen by using a type of session. Here we are saying that dear type script, I know you think this is a 0.2 D, but trust me, it is a 0.3 D and TypeScript being a nice companion happily agrees with you and trusts you completely. However, this single assertion is sometimes not enough. Let's look at an example.

01:06

We know that a 0.3 cannot be assigned to a person as there are no common members between them. The same is true that we cannot assign a person to a 0.3 because of this bidirectional incompatibility. If you try to assign a person to a 0.3, even with a single assertion, TypeScript will still complain. And if you hover over the error message, you can see that TypeScript is saying that this is probably a mistake. But if you still want to do something like this, then add an additional assertion to unknown. And if we do this process of first asserting to unknown

01:40

and then asserting to 0.3 D, indeed cript trust us and does not check us with any errors. Now, from our lesson on unknown, we can understand why this is working. Unknown is allowed at least one way. Compatibility with all types. That is a variable of all types can be assigned to a variable of type unknown. So anything can be assigned to unknown. For example, a person and an unknown can be asserted as anything, for example, a 0.3 D. So that is how our double assertion works.

02:12

And it goes without saying, use it with caution and avoid it as much as possible, but it might help you migrate some JavaScript code.

Professional TypeScript Masterclass

Professional TypeScript Masterclass

3.Primitive Types

4.Instance Types

5.Arrays And Tuples

7.const declarations

8.Function Types

9.Structural Typing

10.Classes in TypeScript

11.Target Compiler Option

13.Special Types any And unknown

14.JavaScript to TypeScript

15.Frontend Projects

16.Type Assertions

17.Type Casting

19.Type Declarations

20.Creating NPM packages

22.Running in NodeJS

23.Lexical this

24.readonly Modifier

26.Literal Types

27.Type Narrowing

28.Discriminated Unions

29.Class Parameter Properties

30.Strict Compiler Option

31.null vs undefined

32.Intersection Types

33.Optional Modifier

34.Non Null Assertion Operator

36.Interface Declaration Merging

37.Types vs Interfaces

39.implements Keyword

40.Definite Assignment Assertion

41.User Defined Type Guards

42.Assertion Functions

43.Function Overloading

44.Call Signatures

45.Abstract Classes

46.Index Signatures

47.Readonly Arrays and Tuples

48.Double Assertions

49.const Assertions

50.this Parameter

51.Generic Constraints

52.typeof Type Operator

53.Lookup Types

54.keyof Type Operator

55.Conditional Types

56.Contitional Types with Unions and never

57.infer Keyword and `ReturnType<T>`

58.Mapped Types

59.Mapped Type Modifiers

60.Template Literal Type

64.Record<K, T>

65.Project References

66.undefined vs. optional

67.satisfies Operator

68.PropertyKey Type

71.String Manipulation Types

72.Mapped Types as Clauses

73.Union vs Intersection Mental Model

74.Enums are Bad