تأكيدات const باستخدام as const

تأكيد as const

const point = { x: 10, y: 20 } as const;
// type: { readonly x: 10; readonly y: 20 }
console.log(point.x, point.y);

من دون as const

const a = { x: 10 };       // x: number, mutable
const b = { x: 10 } as const; // x: 10, readonly
a.x = 99; // allowed
console.log(a.x, b.x);

تثبيت الخصائص كقيم حرفية

const config = {
  env: 'production',
  retries: 3
} as const;
// env: 'production', retries: 3
console.log(config.env, config.retries);

readonly يمنع التغيير

const settings = { mode: 'dark' } as const;
console.log(settings.mode);
// settings.mode = 'light'; // Error: readonly property

تأكيدات const على المصفوفات

const colors = ['red', 'green', 'blue'] as const;
// type: readonly ['red', 'green', 'blue']
console.log(colors[0], colors.length);

لا تغيير في المصفوفات للقراءة فقط

const days = ['Mon', 'Tue', 'Wed'] as const;
console.log(days.join(', '));
// days.push('Thu'); // Error: push does not exist on readonly tuple

اشتقاق اتحاد باستخدام typeof arr[number]

const ROLES = ['admin', 'editor', 'viewer'] as const;
type Role = typeof ROLES[number];
// 'admin' | 'editor' | 'viewer'
const r: Role = 'editor';
console.log(r);

مصدر واحد للحقيقة

const SIZES = ['S', 'M', 'L', 'XL'] as const;
type Size = typeof SIZES[number];

function pick(s: Size): void {
  console.log('Picked', s);
}
pick('M');

تأكيدات const على Tuples

const pair = [1, 'one'] as const;
// type: readonly [1, 'one']
console.log(pair[0], pair[1]);

سلوك readonly العميق

const data = {
  name: 'app',
  tags: ['a', 'b']
} as const;
// tags is readonly ['a', 'b']
console.log(data.name, data.tags[1]);

تأكيدات const للتعدادات

const Status = {
  Idle: 'idle',
  Active: 'active'
} as const;
type Status = typeof Status[keyof typeof Status];
const s: Status = Status.Active;
console.log(s);

تحقق سريع

مراجعة: as const

const LEVELS = ['low', 'mid', 'high'] as const;
type Level = typeof LEVELS[number];
const chosen: Level = 'mid';
console.log(chosen);