Differentiating between array in indexable type in TypeScript

Question

I'm using Object.keys to iterate over an object and I want to execute some logic when the type is an Array. I had even added a check to see whether the value is an array but still TypeScript doesn't infer the value as an array. Is there any way to solve this?

type Test {
  length: number[];
  name: string;
  [key: string]: number[] | string;
}

const test:Test = {
  length: [10, 20 ,30],
  name: 'test'
}

Object.keys(test).map(key => {
  if(Array.isArray(test[key])) {
    test[key].map(i => i)
  }
})

Playground

Heretic Monkey · Accepted Answer · 2020-10-21 17:15:31Z

3

TypeScript is having a hard time keeping track of the discriminated type of Test[string] after the check.

TypeScript can do this with local variables:

if (Array.isArray(someObj)) { someObj.map(() => /*...*/ }

Or explicit properties:

if (Array.isArray(someObj.myArr)) { someObj.myArr.map(() => /*...*/ }

But when the index type is something generic like string, it doesn't quite have enough info for the compiler to figure this inference out.

This has been discussed by the TypeScript team if you want to read more about this. The issue is currently open. Maybe it will be supported in the future, as it does look like the compiler should be able to figure this one out, in theory. (Thanks for the layup @jcalz)

The simplest solution is to change your code into one of the above forms. I would suggest storing a reference to the property value in a local variable and then testing/using that. This allows TypeScript to track type narrowing on that variable.

Object.keys(test).map(key => {
  const maybeArray = test[key]
  if(Array.isArray(maybeArray)) {
    maybeArray.map(i => i)
  }
})

Playground

edited Oct 21, 2020 at 17:15

Heretic Monkey

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answered Oct 21, 2020 at 16:34

Alex Wayne

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2 Comments

jcalz Over a year ago

Rather than post the answer I'm working on, I'll just ask you to include a link to microsoft/TypeScript#10530, the issue about the inability to use control flow analysis to narrow the type of an object's property if the index is a variable like test[key].

Alex Wayne Over a year ago

Thanks! Added. Your answer probably would have been better. They usually are :)

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