接口与类型

接口和类型用于描述对象和原语的类型。接口和类型通常可以互换使用，并且通常提供类似的功能。通常由程序员选择自己的偏好。

然而，接口只能描述创建这些对象的对象和类。因此，必须使用类型来描述字符串和数字等原语。

下面是接口和类型之间的两个区别的示例：

// 1. Declaration merging (interface only)

// This is an extern dependency which we import an object of
interface externDependency { x: number, y: number; }
// When we import it, we might want to extend the interface, e.g. z:number
// We can use declaration merging to define the interface multiple times
// The declarations will be merged and become a single interface
interface externDependency { z: number; }
const dependency: externDependency = {x:1, y:2, z:3}

// 2. union types with primitives (type only)

type foo = {x:number}
type bar = { y: number }
type baz = string | boolean;

type foobarbaz = foo | bar | baz; // either foo, bar, or baz type

// instances of type foobarbaz can be objects (foo, bar) or primitives (baz)
const instance1: foobarbaz = {y:1} 
const instance2: foobarbaz = {x:1} 
const instance3: foobarbaz = true 

https://www.typescriptlang.org/docs/handbook/advanced-types.html

一个区别是，接口创建了一个新名称，该名称在任何地方都可以使用。键入别名不会创建新名称-例如，错误消息不会使用别名。

演示递归地重写Object-Literal类型和接口而不是类成员/财产/函数的能力。

此外，当Record<any，string|number>由于接口等原因而无法工作时，如何区分和键入检查差异以及解决上述问题的方法也可以解决。这将允许对猫鼬类型进行以下简化：https://github.com/wesleyolis/mongooseRelationalTypesmongooseRelationalTypes、DeepPopulate、populate

此外，还有一系列其他方法来实现高级类型泛型和类型推理，以及围绕它的速度怪癖，这些都是一些小技巧，可以通过多次试验和错误来获得正确的结果。

打字游戏场：单击此处查看现场游戏场地中的所有示例

    class TestC {
        constructor(public a: number, public b: string, private c: string) {
    
        }
    }
    
    class TestD implements Record<any, any> {
    
        constructor(public a: number, public b: string, private c: string) {
    
        }
    
        test() : number {
            return 1;
        }
    }
    
    type InterfaceA = {
         a: string,
        b: number,
        c: Date
        e: TestC,
        f: TestD,
        p: [number],
        neastedA: {
            d: string,
            e: number
            h: Date,
            j: TestC
            neastedB: {
                d: string,
                e: number
                h: Date,
                j: TestC
            }
        }
    }


    type TCheckClassResult = InterfaceA extends Record<any, unknown> ? 'Y': 'N' // Y

    const d = new Date();
    type TCheckClassResultClass = typeof d extends Record<any, unknown> ? 'Y': 'N'      // N

    const metaData = Symbol('metaData');
    type MetaDataSymbol = typeof metaData;

    // Allows us to not recuse into class type interfaces or traditional interfaces, in which properties and functions become optional.
    type MakeErrorStructure<T extends Record<any, any>> = {
        [K in keyof T] ?: (T[K] extends Record<any, unknown> ?         MakeErrorStructure<T[K]>: T[K] & Record<MetaDataSymbol, 'customField'>)
    }

    type MakeOptional<T extends Record<any, any>> = {
        [K in keyof T] ?: T[K] extends Record<any, unknown> ? MakeOptional<T[K]> : T[K]
    }

    type RRR = MakeOptional<InterfaceA>
    const res  = {} as RRR;

    const num = res.e!.a; // type == number
    const num2 = res.f!.test(); // type == number

使特定形状的递归形状或键递归

    type MakeRecusive<Keys extends string, T> = {
        [K in Keys]: T & MakeRecusive<K, T>
      } & T
  
    type MakeRecusiveObectKeys<TKeys extends string, T> = {
        [K in keyof T]: K extends TKeys ? T[K] & MakeRecusive<K, T[K]>: T[K]
    }

如何为记录类型应用类型约束，该约束可以验证诸如鉴别器之类的接口：

    type IRecordITypes = string | symbol | number;
    
    // Used for checking interface, because Record<'key', Value> excludeds interfaces
    type IRecord<TKey extends IRecordITypes, TValue> = {
        [K in TKey as `${K & string}`] : TValue
    } 


    // relaxies the valiation, older versions can't validate.
    // type IRecord<TKey extends IRecordITypes, TValue> = {
    //     [index: TKey] : TValue
    // } 

    
    type IRecordAnyValue<T extends Record<any,any>, TValue> = {
        [K in keyof T] : TValue
    }    

    interface AA {
        A : number,
        B : string
    }

    interface BB {
        A: number,
        D: Date
    }

    // This approach can also be used, for indefinitely recursive validation like a deep populate, which can't determine what validate beforehand.
    interface CheckRecConstraints<T extends IRecordAnyValue<T, number | string>> {
    }

    type ResA = CheckRecConstraints<AA> // valid

    type ResB = CheckRecConstraints<BB> // invalid

Alternative for checking keys:

    type IRecordKeyValue<T extends Record<any,any>, TKey extends IRecordITypes, TValue> = 
    {
        [K in keyof T] : (TKey & K) extends never ? never : TValue
    } 
    
    // This approach can also be used, for indefinitely recursive validation like a deep populate, which can't determine what validate beforehand.
    interface CheckRecConstraints<T extends IRecordKeyValue<T, number | string, number | string>> {
        A : T
    }

    type UUU = IRecordKeyValue<AA, string, string | number>

    type ResA = CheckRecConstraints<AA> // valid

    type ResB = CheckRecConstraints<BB> // invalid

然而，使用鉴别器的示例，对于速度，我宁愿使用字面上定义每个要记录的键，然后传递来生成混合值，因为使用更少的内存，而且比这种方法更快。

    type EventShapes<TKind extends string> = IRecord<TKind, IRecordITypes> | (IRecord<TKind, IRecordITypes> & EventShapeArgs)

    type NonClassInstance = Record<any, unknown>
    type CheckIfClassInstance<TCheck, TY, TN> = TCheck extends NonClassInstance ? 'N' : 'Y'

    type EventEmitterConfig<TKind extends string = string, TEvents extends EventShapes<TKind> = EventShapes<TKind>, TNever = never> = {
        kind: TKind
        events: TEvents
        noEvent: TNever
    }

    type UnionDiscriminatorType<TKind extends string, T extends Record<TKind, any>> = T[TKind]

    type PickDiscriminatorType<TConfig extends EventEmitterConfig<any, any, any>,
        TKindValue extends string,
        TKind extends string = TConfig['kind'],        
        T extends Record<TKind, IRecordITypes> & ({} | EventShapeArgs) = TConfig['events'],
        TNever = TConfig['noEvent']> = 
            T[TKind] extends TKindValue 
            ? TNever
            : T extends IRecord<TKind, TKindValue>
                ? T extends EventShapeArgs
                    ? T['TArgs']
                    : [T]
                : TNever        

    type EventEmitterDConfig = EventEmitterConfig<'kind', {kind: string | symbol}, any>
    type EventEmitterDConfigKeys = EventEmitterConfig<any, any> // Overide the cached process of the keys.

    interface EventEmitter<TConfig extends EventEmitterConfig<any, any, any> = EventEmitterDConfig,
                TCacheEventKinds extends string = UnionDiscriminatorType<TConfig['kind'], TConfig['events']>
                > {
      on<TKey extends TCacheEventKinds, 
                    T extends Array<any> = PickDiscriminatorType<TConfig, TKey>>(
                        event: TKey, 
                        listener: (...args: T) => void): this;

     emit<TKey extends TCacheEventKinds>(event: TKey, args: PickDiscriminatorType<TConfig, TKey>): boolean;
    }

用法示例：

    interface EventA {
        KindT:'KindTA'
        EventA: 'EventA'
    }

    interface EventB {
        KindT:'KindTB'
        EventB: 'EventB'
    }

    interface EventC {
        KindT:'KindTC'
        EventC: 'EventC'
    }

    interface EventArgs {
        KindT:1
        TArgs: [string, number]    
    }
    const test :EventEmitter<EventEmitterConfig<'KindT', EventA | EventB | EventC | EventArgs>>;

    test.on("KindTC",(a, pre) => {
        
    })

更好的方法来区分类型并从映射中选择类型以缩小范围，这通常会导致更快的性能和更少的类型操作开销，并允许改进缓存。与前面的示例相比。

    type IRecordKeyValue<T extends Record<any,any>, TKey extends IRecordITypes, TValue> = 
    {
        [K in keyof T] : (TKey & K) extends never ? never : TValue
    } 

    type IRecordKeyRecord<T extends Record<any,any>, TKey extends IRecordITypes> = 
    {
        [K in keyof T] : (TKey & K) extends never ? never : T[K] // need to figure out the constrint here for both interface and records.
    } 
    
    type EventEmitterConfig<TKey extends string | symbol | number, TValue, TMap extends IRecordKeyValue<TMap, TKey, TValue>> = {
        map: TMap
    }

    type PickKey<T extends Record<any,any>, TKey extends any> = (T[TKey] extends Array<any> ? T[TKey] : [T[TKey]]) & Array<never>

    type EventEmitterDConfig = EventEmitterConfig<string | symbol, any, any>


    interface TDEventEmitter<TConfig extends EventEmitterConfig<any, any, TConfig['map']> = EventEmitterDConfig,
        TMap = TConfig['map'],
        TCacheEventKinds = keyof TMap
    > {
        
        on<TKey extends TCacheEventKinds, T extends Array<any> = PickKey<TMap, TKey>>(event: TKey, 
            listener: (...args: T) => void): this;

        emit<TKey extends TCacheEventKinds, T extends Array<any> = PickKey<TMap, TKey>>(event: TKey, ...args: T): boolean;
    }
   
    type RecordToDiscriminateKindCache<TKindType extends string | symbol | number, TKindName extends TKindType, T extends IRecordKeyRecord<T, TKindType>> = {
        [K in keyof T] : (T[K] & Record<TKindName, K>)
    }

    type DiscriminateKindFromCache<T extends IRecordKeyRecord<T, any>> = T[keyof T]

用法示例：

    
    interface EventA {
        KindT:'KindTA'
        EventA: 'EventA'
    }

    interface EventB {
        KindT:'KindTB'
        EventB: 'EventB'
    }

    interface EventC {
        KindT:'KindTC'
        EventC: 'EventC'
    }

    type EventArgs = [number, string]

    type Items = {
        KindTA : EventA,
        KindTB : EventB,
        KindTC : EventC
        //0 : EventArgs,
    }

    type DiscriminatorKindTypeUnionCache = RecordToDiscriminateKindCache<string 
    //| number,
    "KindGen", Items>;

    type CachedItemForSpeed = DiscriminatorKindTypeUnionCache['KindTB']

    type DiscriminatorKindTypeUnion = DiscriminateKindFromCache<DiscriminatorKindTypeUnionCache>;

    function example() {
        
        const test: DiscriminatorKindTypeUnion;
        switch(test.KindGen) {
            case 'KindTA':
                test.EventA
                break;
            case 'KindTB':
                test.EventB
                break;
            case 'KindTC':
                test.EventC

            case 0:
                test.toLocaleString

        }
    }


    type EmitterConfig = EventEmitterConfig<string 
    //| number
    , any, Items>;

    const EmitterInstance :TDEventEmitter<EmitterConfig>;

    EmitterInstance.on("KindTB",(a, b) => {
        
        a.

    })

索引的差异。

interface MyInterface {
  foobar: string;
}

type MyType = {
  foobar: string;
}

const exampleInterface: MyInterface = { foobar: 'hello world' };
const exampleType: MyType = { foobar: 'hello world' };

let record: Record<string, string> = {};

record = exampleType;      // Compiles
record = exampleInterface; // Index signature is missing

相关问题：类型中缺少索引签名（仅在接口上，而不是在类型别名上）

因此，如果您想为对象编制索引，请考虑这个示例

看看这个问题和这个关于违反利斯科夫原则的问题

评估中的差异

当FirstLevelType为接口时，请查看ExtendeFirst的结果类型

/**
 * When FirstLevelType is interface 
 */

interface FirstLevelType<A, Z> {
    _: "typeCheck";
};

type TestWrapperType<T, U> = FirstLevelType<T, U>;


const a: TestWrapperType<{ cat: string }, { dog: number }> = {
  _: "typeCheck",
};

// {  cat: string; }
type ExtendFirst = typeof a extends FirstLevelType<infer T, infer _>
    ? T
    : "not extended";

当FirstLevelType为类型时，请查看ExtendeFirst的结果类型：

/**
 * When FirstLevelType is type
 */
type FirstLevelType<A, Z>= {
    _: "typeCheck";
};

type TestWrapperType<T, U> = FirstLevelType<T, U>;


const a: TestWrapperType<{ cat: string }, { dog: number }> = {
  _: "typeCheck",
};

// unknown
type ExtendFirst = typeof a extends FirstLevelType<infer T, infer _>
    ? T
    : "not extended";

文档中指出的关键区别在于，可以重新打开接口以添加新属性，但不能重新打开类型别名以添加新的属性，例如：

这没问题

interface x {
  name: string
}

interface x {
  age: number
}

这将抛出错误Duplicate identifier y

type y = {
  name: string
}

type y = {
  age: number
}

除此之外，接口和类型别名类似。

2021 3月更新：更新的TypeScript手册（也在nju-clc中提到下面的答案）有一节“接口与类型别名”，解释了区别。

原始答案（2016）

根据（现已存档）TypeScript语言规范：

与总是引入命名对象类型的接口声明不同，类型别名声明可以引入任何类型的名称，包括基元类型、联合类型和交集类型。

该规范还提到：

接口类型与对象类型的类型别名有许多相似之处但是由于接口类型提供了更多的功能通常优选键入别名。例如，接口类型接口点{x： 数量；y： 数量；}可以写成类型别名类型点={x： 数量；y： 数量；};但是，这样做意味着失去以下功能：接口可以在extends或implements子句中命名，但对象类型文本的类型别名不能在TS 2.7之后为true。接口可以有多个合并声明，但对象类型文本的类型别名不能。