TypeScript 函数根据输入参数返回类型 [英] TypeScript function return type based on input parameter

问题描述

我有几个不同的接口和对象，每个接口和对象都有一个 type 属性.假设这些是存储在 NoSQL 数据库中的对象.如何根据输入参数 type 创建具有确定性返回类型的通用 getItem 函数?

interface Circle {类型:圆圈"；半径:数量；}界面广场{类型:正方形"；长度:数量；}const 形状:(圆形 | 方形)[] = [{ 类型:圆"，半径:1 }，{ 类型:圆"，半径:2 }，{ 类型:正方形"，长度:10 }];函数getItems(类型:圆形"|方形"){返回shapes.filter(s => s.type == type);//将此视为来自数据库的项目//我希望这个函数的返回类型是//基于作为参数提供的 `type` 值的确定性.}const circles = getItems("circle");for (const circle of circle) {控制台日志(圆.半径)；^^^^^^}

<块引用>

圆"类型上不存在半径"属性 |方'.

解决方案

条件类型:

interface Circle {类型:圆圈"；半径:数量；}界面广场{类型:方形"；长度:数量；}type TypeName = "circle";|正方形"；类型 ObjectType=T延伸圆"?圆圈 :T延伸正方形"?正方形 :绝不;const 形状:(圆形 | 方形)[] = [{ 类型:圆"，半径:1 }，{ 类型:圆"，半径:2 }，{ 类型:正方形"，长度:10 }];function getItems(type: T) : ObjectType[] {返回 shape.filter(s => s.type == type) 作为 ObjectType[];}const circles = getItems("circle");for (const circle of circle) {控制台日志(圆.半径)；}

感谢 Silvio 为我指明了正确的方向.

I have a few different interfaces and objects that each have a type property. Let's say these are objects stored in a NoSQL db. How can I create a generic getItem function with a deterministic return type based on its input parameter type?

interface Circle {
    type: "circle";
    radius: number;
}

interface Square {
    type: "square";
    length: number;
}

const shapes: (Circle | Square)[] = [
    { type: "circle", radius: 1 },
    { type: "circle", radius: 2 },
    { type: "square", length: 10 }];

function getItems(type: "circle" | "square") {
    return shapes.filter(s => s.type == type);
    // Think of this as items coming from a database
    // I'd like the return type of this function to be
    // deterministic based on the `type` value provided as a parameter. 
}

const circles = getItems("circle");
for (const circle of circles) {
    console.log(circle.radius);
                       ^^^^^^
}

Property 'radius' does not exist on type 'Circle | Square'.

解决方案

Conditional Types to the rescue:

interface Circle {
    type: "circle";
    radius: number;
}

interface Square {
    type: "square";
    length: number;
}

type TypeName = "circle" | "square"; 

type ObjectType<T> = 
    T extends "circle" ? Circle :
    T extends "square" ? Square :
    never;

const shapes: (Circle | Square)[] = [
    { type: "circle", radius: 1 },
    { type: "circle", radius: 2 },
    { type: "square", length: 10 }];

function getItems<T extends TypeName>(type: T) : ObjectType<T>[]  {
    return shapes.filter(s => s.type == type) as ObjectType<T>[];
}

const circles = getItems("circle");
for (const circle of circles) {
    console.log(circle.radius);
}

Thanks Silvio for pointing me in the right direction.

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