close_for_modification.md

Prefer immutability

TypeScript's type system allows you to mark individual properties on an interface/class as readonly. This allows you to work in a functional way (an unexpected mutation is bad).
For more advanced scenarios there is a built-in type Readonly that takes a type T and marks all of its properties as readonly using mapped types (see mapped types).

Bad:

interface Config {
  host: string;
  port: string;
  db: string;
}

Good:

interface Config {
  readonly host: string;
  readonly port: string;
  readonly db: string;
}

For arrays, you can create a read-only array by using ReadonlyArray.
It doesn't allow changes such as push() and fill(), but can use features such as concat() and slice() that do not change the array's value.

Bad:

const array: number[] = [1, 3, 5];
array = []; // error
array.push(100); // array will be updated

Good:

const array: ReadonlyArray<number> = [1, 3, 5];
array = []; // error
array.push(100); // error

Declaring read-only arguments in TypeScript 3.4 is a bit easier.

function hoge(args: readonly string[]) {
  args.push(1); // error
}

Prefer const assertions for literal values.

Bad:

const config = {
  hello: 'world'
};
config.hello = 'world'; // value is changed

const array = [1, 3, 5];
array[0] = 10; // value is changed

// writable objects is returned
function readonlyData(value: number) {
  return { value };
}

const result = readonlyData(100);
result.value = 200; // value is changed

Good:

// read-only object
const config = {
  hello: 'world'
} as const;
config.hello = 'world'; // error

// read-only array
const array = [1, 3, 5] as const;
array[0] = 10; // error

// You can return read-only objects
function readonlyData(value: number) {
  return { value } as const;
}

const result = readonlyData(100);
result.value = 200; // error