typescript 实现函数重载的方式

函数重载

JavaScript并没有提供函数重载

TypeScript提供的函数重载，仅仅用作提示效果，实现还需手动

函数重载简单点说就是，同一个函数的不同实现方式，根据传入的参数的类型或者长度，决定最终调用哪一个实现

最终效果,TypeScript的类型校验也会变化

code

创建函数重载函数

根据参数的类型，调用不同的实现，如果没有对应的实现，则报错。

export function createOverload () {

    const map: Map<string, Function> = new Map();

    const overload = ( ...args: any[] ) => {

        const key = args.map( it => typeof it ).join( " " );

        const fn = map.get( key );

        if ( !fn ) throw new Error( "No overload function matched" );

        return fn( args );
    };

    overload.addImpl = function ( args: Array<"number" | "string" | "boolean" | "function" | "object" | "symbol" | "undefined" | "bigint">, fn: Function ) {

        if ( typeof fn !== "function" ) throw new Error( "last argument must be a function" );

        const key = args.join( " " );

        map.set( key, fn );

    };

    return overload;

}

使用

    const overload = createOverload()
    
    overload.addImpl(["string","number"],()=>{
        console.log("string number")
    })
    overload.addImpl(["number","string"],()=>{
        console.log("number string")
    })
    
    overload("yang jun",18) // console.log("string number")
    overload(18,"yang jun") // console.log("number string")

封装一层，因为上述使用没有代码提示，离了代码提示活不下去了

创建抽象类。

在JS中创建抽象类方法，在construct中执行 if( new.target === Overload ) throw new Error("不允许直接new")

在下述实现中，转为 es6 类实现，新增了重载函数的映射表，用于继承类的多个函数的重载。

export abstract class Overload {

    private overloads = new Map<string, Function>();

    protected addImpl ( name: string, args: Array<"string" | "number" | "bigint" | "boolean" | "symbol" | "undefined" | "object" | "function">, fn: Function ) {

        let overload = null;

        if ( this.overloads.has( name ) ) {

            overload = this.overloads.get( name );

        } else {

            overload = createOverload();

            this.overloads.set( name, overload );

        }

        overload.addImpl( args, fn );
    };

    protected getOverload ( name: string ): Function | undefined {

        if ( this.overloads.has( name ) ) return this.overloads.get( name );

        throw new Error( "No overload matched" );

    }

}

使用

非常舒服，再也不用自己在函数中写一串的 if 了

import { Overload } from "./utils";

class Test extends Overload {

    constructor () {

        super();

        this.addImpl( "getInfo", [ "boolean", "number", "string" ], () => {
            console.log( "boolean", "number", "string" );
        } );
        this.addImpl( "getInfo", [ "number", "string", "boolean" ], () => {
            console.log( "number", "string", "boolean" );
        } );
        this.addImpl( "getInfo", [ "string", "number", "boolean" ], () => {
            console.log( "string", "number", "boolean" );
        } );

    }

    getInfo ( age: number, name: string, isHandsome: boolean ): Object;
    getInfo ( isHandsome: boolean, age: number, name: string ): Object;
    getInfo ( name: string, age: number, isHandsome: boolean ): Object;

    getInfo (): Object {
        const overload = this.getOverload( "getInfo" );
        return overload( ...arguments );
    }

    getName ( name: string ) {

    }

}

const test = new Test();

test.getInfo( 18, "yang jun", true );  //    console.log( "number", "string", "boolean" );

test.getInfo( "yang jun", 18, true );  //  console.log( "string", "number", "boolean" );

test.getInfo( true, 18, "yang jun" ); //   console.log( "boolean", "number", "string" );