Typescript - change value of a dictionary with a key variable

Question

I have an object:

const sampleExpenseData = {
  Time: "11-12-19",
  Meals: 5130,
  Pantry: 10,
  Living: 10,
  Others: 0,
  Total: 74
}

I would like to define a function such that it will update the value of a specific key, what I have so far is:

const updateValueInDict = (
  obj: typeof sampleExpenseData,
  key: keyof typeof sampleExpenseData,
  upDatedValue: string | number
): void => {
  obj[key] = upDatedValue  // this line throws error
}

The error says:

Type 'string | number' is not assignable to type 'never'.

Type 'string' is not assignable to type 'never'.ts(2322)

TIA!!!

kaya3 · Accepted Answer · 2019-12-20 23:16:29Z

2

The problem here is that if you're assigning a value to an arbitrary key, then that value must have a type which is assignable to every key you might be assigning it to. That means the value's type has to be string & number in order to be assignable to both string and number property types. The intersection string & number is never.

The solution is to use a generic type, so that upDatedValue is constrained to have the right type depending on the type of key:

const updateValueInDict = <K extends keyof typeof sampleExpenseData>(
  obj: typeof sampleExpenseData,
  key: K,
  upDatedValue: (typeof sampleExpenseData)[K]
): void => {
  obj[key] = upDatedValue
}

This also means that the arguments can be type-checked more strictly when the function is called.

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answered Dec 20, 2019 at 23:16

kaya3

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5 Comments

Kiskae Over a year ago

You might want to consider changing typeof sampleExpenseData into a generic parameter too.

kaya3 Over a year ago

Yes, that simplifies the declaration substantially, and is probably better; I just wanted to change the code as little as possible in order to demonstrate the idea. That said, there is one downside which might apply in some cases, if you want to be able to specify the type argument explicitly like updateValueInDict<'foo'>(bar, baz, quz) then having a second type argument could make that more annoying.

Yao Over a year ago

Thanks, but one more thing: I would also like to convert some of the keys to number, it throws error when I do if(!isNaN(number(updatedValue))) text = Number(updatedValue). Type 'number' is not assignable to type 'never'

kaya3 Over a year ago

That doesn't seem to have anything to do with this code. I suggest asking it as a separate question, and you will need to include a full minimal reproducible example.

Yao Over a year ago

Cool as! Thank you very much

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