I am new to TypeScript and have just started migrating my application from JavaScript to TypeScript.

1. First function: calcAvg, takes an array of numbers and returns their average.


2. Second function: avgRoasDiscrepancy, takes two arrays, if the two arrays are of equal length and with no null values, it will subtract each element from one list with the element in the same index from the second array.

The issue is in the following map function (see full code below):

const discArray = clientRoasArray.map((value: number, index: number) => value - roasArray[index]);

I get the following error:

Argument of type '(value: number, index: number) => number' is not assignable to parameter of type '(value: number | null, index: number, array: (number | null)[]) => number'.

Types of parameters 'value' and 'value' are incompatible.

Type 'number | null' is not assignable to type 'number'.

Type 'null' is not assignable to type 'number'

I don't know how to "convince" TypeScript that both value and roasArray[index] are in fact numbers and not nulls.

Here is the complete code :

interface DailyData {

  id: number

  clientCost:number

  conValue:number

  clientConvValue:number

  margin: number



}



const calcAvg = (dataList: number[]) => {

  const reducer = (accumulator: number, curr: number) => accumulator + curr;

  const dataSum = dataList.reduce(reducer, 0);

  return dataSum / dataList.length;

};





const avgRoasDiscrepancy = (advertiserStats: DailyData[]) => {

  const clientRoasArray = advertiserStats.map((obj: DailyData) => obj.clientCost > 0 ? obj.clientConvValue/obj.clientCost: null);

  const roasArray = advertiserStats.map((obj: DailyData) => obj.clientCost > 0 ? obj.conValue/obj.clientCost: null);

  if (clientRoasArray.length === 0 || roasArray.length === 0) {

    return 'no data'

  }

  if (!!clientRoasArray.every((el: number|null) => el!== null) || (!roasArray.every((el: number|null) => el!== null) )) {

    //if one of the arrays has null values don't calculate

    return 'no data';

  }

  if (clientRoasArray.length === roasArray.length) {

    const discArray = clientRoasArray.map((value: number, index: number) => value - roasArray[index]);

    return calcAvg(discArray as number[]);

  }

  return 'no data';



};