数组元素 "possibly undefined" 错误的类型保护

Question

TSC 给出一个错误，该错误作为注释添加到代码中。

tsconfig: "noUncheckedIndexedAccess": true

type Tfactors = [number, 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, 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) {
    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。我可以在不声明 pocketValue 和 factor 变量并且不使用 ! 使 TSC 静音的情况下解决这个问题吗？

Answer 1

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

One consequence of using --noUncheckedIndexedAccess is that indexing into an array is also more strictly checked, even in a bounds-checked loop.

Under this new mode, every property access (like foo.bar) or indexed access (like foo["bar"]) is considered potentially undefined.

解决方案：

如果您需要访问那个属性，您要么必须先检查它是否存在，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]!;

Answer 2

如果您不想使用!来屏蔽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, 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;
}

Answer 3

Can I solve this problem without declaring pocketValue and factor variables [or] without use ! to mute TSC.

不，不幸的是你不能。

数组元素 "possibly undefined" 错误的类型保护

Type guard for array element's "possibly undefined" error

arrays

typescript

undefined

typeguards