类型守卫
11 methods在条件分支中收窄类型的运行时检查。
Partial<T>使用 typeof 运算符收窄原始类型。可识别：'string'、'number'、'boolean'、'bigint'、'symbol'、'undefined'、'object'、'function'。
Parameters
| Name | Type | Description |
|---|---|---|
| T | type | 要收窄的值。 |
Returns
type
Example
typescript
interface User { id: number; name: string; }
type UserPatch = Partial<User>;
// { id?: number; name?: string }
const patch: UserPatch = { name: 'Alice' };Required<T>通过检查原型链收窄为实例类型。
Parameters
| Name | Type | Description |
|---|---|---|
| T | type | 要收窄的值。 |
Returns
type
Example
typescript
interface Props { a?: number; b?: string; }
type RequiredProps = Required<Props>;
// { a: number; b: string }Readonly<T>通过检查属性是否存在来收窄联合类型。
Parameters
| Name | Type | Description |
|---|---|---|
| key | type | 要检查的属性名。 |
Returns
type
Example
typescript
interface Config { url: string; }
type FrozenConfig = Readonly<Config>;
const c: FrozenConfig = { url: '/' };
c.url = '/x'; // Error: readonlyRecord<K, V>用户定义的类型守卫。'x is T' 返回类型谓词在函数返回 true 时触发收窄。
Parameters
| Name | Type | Description |
|---|---|---|
| K | type | 要测试的值。 |
| V | type | Value type. |
Returns
type
Example
typescript
type Role = 'admin' | 'user';
const perms: Record<Role, string[]> = {
admin: ['read', 'write'],
user: ['read'],
};Pick<T, K>Pick only the keys K from T.
Parameters
| Name | Type | Description |
|---|---|---|
| T | type | Source type. |
| K | type | Keys to pick (union of keyof T). |
Returns
type
Example
typescript
interface User { id: number; name: string; email: string; }
type UserSummary = Pick<User, 'id' | 'name'>;
// { id: number; name: string }Omit<T, K>Construct a type with all properties of T except those in K.
Parameters
| Name | Type | Description |
|---|---|---|
| T | type | Source type. |
| K | type | Keys to omit. |
Returns
type
Example
typescript
interface User { id: number; name: string; email: string; }
type CreateUserDTO = Omit<User, 'id'>;
// { name: string; email: string }Exclude<T, U>Exclude from union T all types assignable to U.
Parameters
| Name | Type | Description |
|---|---|---|
| T | type | Source union. |
| U | type | Types to exclude. |
Returns
type
Example
typescript
type T = string | number | boolean;
type N = Exclude<T, boolean>;
// string | numberExtract<T, U>Extract from union T all types assignable to U.
Parameters
| Name | Type | Description |
|---|---|---|
| T | type | Source union. |
| U | type | Types to extract. |
Returns
type
Example
typescript
type T = string | number | boolean;
type S = Extract<T, string>;
// stringNonNullable<T>Exclude null and undefined from T.
Parameters
| Name | Type | Description |
|---|---|---|
| T | type | Source type. |
Returns
type
Example
typescript
type T = string | null | undefined;
type NN = NonNullable<T>;
// stringReturnType<T>Return the return type of function type T.
Parameters
| Name | Type | Description |
|---|---|---|
| T | type | Function type. |
Returns
type
Example
typescript
function getUser() { return { id: 1, name: 'a' }; }
type User = ReturnType<typeof getUser>;
// { id: number; name: string }Parameters<T>Return a tuple type of the parameter types of function type T.
Parameters
| Name | Type | Description |
|---|---|---|
| T | type | Function type. |
Returns
type
Example
typescript
function add(a: number, b: number) { return a + b; }
type AddArgs = Parameters<typeof add>;
// [number, number]