数组元素“可能未定义”错误的类型保护

问题描述

TSC 给出了一个错误，作为注释添加到代码中。

tsconfig："noUncheckedindexedAccess": true

type Tfactors = [number,number,number];

export default function changeEnough(pocket: Tfactors,bill: number): boolean {
  const coinTodolarFactors: Tfactors = [0.25,0.1,0.5,0.01];
  let pocketTotal = 0;

  for (let i in pocket) {

    if (pocket[i] !== undefined && coinTodolarFactors[i] !== undefined) {
      //Object is possibly 'undefined'.ts(2532)
      pocketTotal += pocket[i] * coinTodolarFactors[i];
    }
  }

  return pocketTotal >= bill;
}

我是这样解决这个问题的：

type Tfactors = [number,0.01];
  let pocketTotal = 0;

  for (let i in pocket) {
    const pocketValue = pocket[i];
    const factor = coinTodolarFactors[i];
    if (pocketValue !== undefined && factor !== undefined) {
      pocketTotal += pocketValue * factor;
    }
  }

  return pocketTotal >= bill;
}

我是 TypeScript 的新手，我不知道这是解决这个问题的最佳方法。我能否在不声明 pocketValue 和 factor 变量且不使用 ! 使 TSC 静音的情况下解决此问题。

解决方法

从文档 Pedantic Index Signature Checks (--noUncheckedIndexedAccess) 中，我们知道

使用 --noUncheckedIndexedAccess 的一个结果是，即使在边界检查循环中，对数组的索引也受到更严格的检查。

在这种新模式下，每个属性访问（如 foo.bar）或索引访问（如 foo["bar"]）都被视为潜在的 undefined。

解决方案：

如果您需要访问该属性，您要么必须首先检查它是否存在，要么使用非空断言运算符（后缀 ! 字符）。

// Checking if it's really there first.
const pocketValue = pocket[i];
const factor = coinToDolarFactors[i];
if (pocketValue !== undefined && factor !== undefined) {
  pocketTotal += pocketValue * factor;
}


// Basically saying "trust me I know what I'm doing"
// with the '!' non-null assertion operator.
pocketTotal += pocket[i]! * coinToDolarFactors[i]!;

,

如果您不想使用 ! 来静音 TSC 或将数组元素分配给新变量，您可以显式键入数组索引，即

type Tfactors = [number,number,number];

export default function changeEnough(pocket: Tfactors,bill: number): boolean {
  const coinToDolarFactors: Tfactors = [0.25,0.1,0.5,0.01];
  let pocketTotal = 0;

  for (let _i in pocket) {
    /* --  type it explicitly here -- */
    let i = _i as Exclude<keyof Tfactors,keyof []>;
    // *if* check is not required if you are confident they are defined at runtime  
    if (pocket[i] !== undefined && coinToDolarFactors[i] !== undefined) {
      //Object is possibly 'undefined'.ts(2532)
      pocketTotal += pocket[i] * coinToDolarFactors[i];
    }
  }

  return pocketTotal >= bill;
}