docs: docs

packages/eslint-plugin/docs/rules/prefer-readonly-parameter-types.md

@@ -0,0 +1,91 @@
+# Requires that function parameters are typed as readonly to prevent accidental mutation of inputs (`prefer-readonly-parameter-types`)
+
+Mutating function arguments can lead to confusing, hard to debug behavior.
+Whilst it's easy to implicitly remember to not modify function arguments, explicitly typing arguments as readonly provides clear contract to consumers.
+This contract makes it easier for a consumer to reason about if a function has side-effects.
+
+## Rule Details
+
+This rule allows you to enforce that function parameters resolve to readonly types.
+A type is considered readonly if:
+
+- it is a primitive type (`string`, `number`, `boolean`, `symbol`, or an enum),
+- it is a function signature type,
+- it is a readonly array type whose element type is considered readonly.
+- it is a readonly tuple type whose elements are all considered readonly.
+- it is an object type whose properties are all marked as readonly, and whose values are all considered readonly.
+
+Examples of **incorrect** code for this rule:
+
+```ts
+function array1(arg: string[]) {} // array is not readonly
+function array2(arg: readonly string[][]) {} // array element is not readonly
+function array3(arg: [string, number]) {} // tuple is not readonly
+function array4(arg: readonly [string[], number]) {} // tuple element is not readonly
+// the above examples work the same if you use ReadonlyArray<T> instead
+
+function object1(arg: { prop: string }) {} // property is not readonly
+function object2(arg: { readonly prop: string; prop2: string }) {} // not all properties are readonly
+function object3(arg: { readonly prop: { prop2: string } }) {} // nested property is not readonly
+// the above examples work the same if you use Readonly<T> instead
+
+interface CustomArrayType extends ReadonlyArray<string> {
+  prop: string; // note: this property is mutable
+}
+function custom1(arg: CustomArrayType) {}
+
+interface CustomFunction {
+  (): void;
+  prop: string; // note: this property is mutable
+}
+function custom2(arg: CustomFunction) {}
+
+function union(arg: string[] | ReadonlyArray<number[]>) {} // not all types are readonly
+```
+
+Examples of **correct** code for this rule:
+
+```ts
+function array1(arg: readonly string[]) {}
+function array2(arg: readonly (readonly string[])[]) {}
+function array3(arg: readonly [string, number]) {}
+function array4(arg: readonly [readonly string[], number]) {}
+// the above examples work the same if you use ReadonlyArray<T> instead
+
+function object1(arg: { readonly prop: string }) {}
+function object2(arg: { readonly prop: string; readonly prop2: string }) {}
+function object3(arg: { readonly prop: { readonly prop2: string } }) {}
+// the above examples work the same if you use Readonly<T> instead
+
+interface CustomArrayType extends ReadonlyArray<string> {
+  readonly prop: string;
+}
+function custom1(arg: CustomArrayType) {}
+
+interface CustomFunction {
+  (): void;
+  readonly prop: string;
+}
+function custom2(arg: CustomFunction) {}
+
+function union(arg: readonly string[] | ReadonlyArray<number[]>) {}
+
+function primitive1(arg: string) {}
+function primitive2(arg: number) {}
+function primitive3(arg: boolean) {}
+function primitive4(arg: unknown) {}
+function primitive5(arg: null) {}
+function primitive6(arg: undefined) {}
+function primitive7(arg: any) {}
+function primitive8(arg: never) {}
+function primitive9(arg: string | number | undefined) {}
+
+function fnSig(arg: () => void) {}
+
+enum Foo { a, b }
+function enum(arg: Foo) {}
+
+function symb1(arg: symbol) {}
+const customSymbol = Symbol('a');
+function symb2(arg: typeof customSymbol) {}
+```