为啥在数组上使用 ?: 运算符时打字稿不需要`undefined`？ [复制]

Posted 2023-03-13

【中文标题】为啥在数组上使用 ?: 运算符时打字稿不需要`undefined`？ [复制]

【英文标题】：Why does typescript not require `undefined` when using the ?: operator on an array? [duplicate]为什么在数组上使用 ?: 运算符时打字稿不需要`undefined`？ [复制]

【发布时间】：2022-01-24 00:10:57

【问题描述】：

type Artifact = 
    a:string


const items : Artifact[] = [];

// this is a syntax error
let z?: Artifact;

// this is an error because you cannot assign undefined.
const b : Artifact = undefined;

// this worked!? but I don't think it should because the 
// value could be undefined
const c : Artifact = items?.[0];

// These work as expected.
const a : Artifact|undefined = undefined;
const d : Artifact|undefined = items?.[0];

Playground

【问题讨论】：

?: 运算符是什么？您的问题是“为什么编译器认为读取的数组永远不会产生undefined？我假设是这样；请参阅this question 及其答案。如果我的假设是错误的并且您要问其他问题，请edit 区分这是另一个问题。祝你好运！

【参考方案1】：

项目在您的代码中不断定义，所以“？”被忽略

我扩展了您的示例以显示所有变体 Playground Link

const items: Artifact[] | undefined = [];
const undefinedItems: Artifact[] | undefined = undefined;
function getItems(): Artifact[] | undefined 
    return [];

const c0 : Artifact = items[0];
const c1 : Artifact = items?.[0];
const c2 : Artifact = undefinedItems?.[0];
//    ^ Type 'undefined' is not assignable to type 'Artifact'
const c3 : Artifact = getItems()[0];
//                    ^ Object is possibly 'undefined'.
const c4 : Artifact = getItems()?.[0];
//    ^ Type 'Artifact | undefined' is not assignable to type 'Artifact'