TypeScript 之前检查的未定义值错误

Question

I created a function that either adds an item to an array or updates an item to a given index if set.

I'm using TypeScript and I found a really strange behavior that I can't seem to understand.

Here is a Playground Link .

This simplified function is fine for TypeScript:

function OKAddOrUpdateFunction(item: string, index?: number) {
    const foo = index !== undefined
        ? Object.assign([], initialArray, { [index]: item }) : [...initialArray, item];
}

Now if I use a const and store either the index is defined or not so I can use it later on:

function NOKAddOrUpdateFunction(item: string, index?: number) {
    const isIndexDefined = index !== undefined;

    const foo = isIndexDefined
        ? Object.assign([], initialArray, { [index]: item }) : [...initialArray, item];
}

TypeScript throws an error pointing at the index inside the Object.assign :

(parameter) index: number | undefined
A computed property name must be of type 'string', 'number', 'symbol', or 'any'.(2464)

And I can't understand why...

Answer 1

Consider such a case

function NOKAddOrUpdateFunction(item: string, index?: number) {
    const isIndexDefined = index !== undefined || true; // always true

    const foo = isIndexDefined
        ? Object.assign([], initialArray, { [index]: item }) : [...initialArray, item];
}

Now even if index is undefined , isIndexDefined will be true so this code will run Object.assign([], initialArray, { [index]: item }) and it will fail, because index of Array can not be undefined. (That is what TS is warning you about).

TS can not make an assumption about your variables before if statement.

In the first case, you check for undefined directly in if like if (index !== undefined) and in this case it is clear for TS that in true case your index is a number.

So possible solutions:

• check for undefined inside if statement (preferred)
• write [index as number], only if you are sure (if someone will change code before if, TS will not detect an error)