TypeScript Special Types

TypeScript has special types that may not refer to any specific type of data.

#JavaScript#TypeScript

18th August 2022

Time to read: 1 min

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Type: any

any is a type that disables type checking and effectively allows all types to be used.

This example does not use any and will throw an error:

let u = true;
u = "string"; // Error: Type 'string' is not assignable to type 'boolean'.
Math.round(u); // Error: Argument of type 'boolean' is not assignable to parameter of type 'number'.

Setting any to the special type any disables type checking:

let v: any = true;
v = "string"; // no error as it can be "any" type
Math.round(v); // no error as it can be "any" type

Type: unknown

unknown is a similar, but safer alternative to any. TypeScript will prevent unknown types from being used:

let w: unknown = 1;
w = "string"; // no error
w = {
  runANonExistentMethod: () => {
    console.log("I think therefore I am");
  }
} as { runANonExistentMethod: () => void}
// How can we avoid the error for the code commented out below when we don't know the type?
// w.runANonExistentMethod(); // Error: Object is of type 'unknown'.
if(typeof w === 'object' && w !== null) {
  (w as { runANonExistentMethod: Function }).runANonExistentMethod();
}
// Although we have to cast multiple times we can do a check in the if to secure our type and have a safer casting

unknown is best used when you don't know the type of data being typed. To add a type later, you'll need to cast it. Casting is when we use the "as" keyword to say property or variable is of the casted type.

Type: never

never effectively throws an error whenever it is defined.

let x: never = true; // Error: Type 'boolean' is not assignable to type 'never'.

Type: undefined & null

undefined and null are types that refer to the JavaScript primitives undefined and null respectively.

let y: undefined = undefined;
let z: null = null;

These types don't have much use unless strictNullChecks is enabled in the tsconfig.json file.