CODEWITHSUNDEEP
reactjstypescript
Yao
Yao

Reputation: 439

Typescript - change value of a dictionary with a key variable

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!!!

Upvotes: 0

Views: 5166

Answers (1)

kaya3
kaya3

Reputation: 51034

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.

Playground Link

Upvotes: 2

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