Identifying data types in arrays using TypeScript type predicates

My current approach involves a function that validates if a variable is an object and not null:

function isRecord(input: any): input is Record<string, any> {
  return input !== null && typeof input === 'object';
}

This type predicate is essential for TypeScript to accept the following code snippet:

if (isRecord(x)) {
  console.log(x["att"]);
}

I also created another function that operates on arrays, but I am encountering an error stating "Object is possibly 'null'":

function areRecords(list: any[]): list is Record<string, any>[] {
  return list.every(element => isRecord(element));
}

if (areRecords(x, y)) {
  console.log(x["att"]);
  console.log(y["att"]);
}

The same error persists even if I remove the "is" keyword:

function areRecords2(list: any[]): boolean {
  return list.every(element => isRecord(element));
}

if (areRecords2([x, y])) {
  console.log(x["att"]);
  console.log(y["att"]);
}

An alternative attempt using rest parameters has also not resolved the issue:

function areRecords3(...list: any[]): boolean {
  return list.every(element => isRecord(element));
}

if (areRecords3(x, y)) {
  console.log(x["att"]);
  console.log(y["att"]);
}

I am seeking guidance on how to correct this. What would be the right way to handle this situation?

Answer №1

When using the array function, it is important to note that the signature indicates the array as a record holder rather than specifying x and y individually.

For instance:

const data = [x, y];
if (areRecords(data)) {
    // TypeScript recognizes data as Record<string, any>[], but does not identify x and y individually due to the function signature.
}

To work around this limitation, you can:

const data = [x, y];
if (areRecords(data)) {
    data.forEach(item => console.log(item["attribute"]));
}

Alternatively:

const attributes = [x, y].filter(isRecord).map(value => value["attribute"]);

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