[英]Type guard for array element's “possibly undefined” error
TSC 给出了一个错误,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;
}
我这样解决了这个问题:
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,我不知道这是解决此问题的最佳方法。 我可以在不声明pocketValue
和factor
变量且不使用的情况下解决这个问题!
静音 TSC。
如果不想用!
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, 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;
}
从文档Pedantic Index Signature Checks (--noUncheckedIndexedAccess) ,我们知道
使用
--noUncheckedIndexedAccess
的一个结果是对数组的索引也会受到更严格的检查,即使在边界检查循环中也是如此。
在这种新模式下,每个属性访问(如
foo.bar
)或索引访问(如foo["bar"]
)都被认为是潜在的undefined
。
解决方案:
如果您需要访问该属性,则必须首先检查其是否存在或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]!;
我可以在不声明pocketValue 和因子变量[或] 不使用的情况下解决这个问题吗? 静音 TSC。
不,很遗憾,你不能。
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