I'm writing a typescript function that accepts a numeric array (i.e., type: number[]) and calculates its mean. In addition, I want to account for when the input array might contain some null values. To this end, I added an argument, that when set to true, tells the function to remove nulls before calculating the mean.

But I can't figure out the proper way to do this, as I can't override the input within the function.

Here's my code for calcMean()

function calcMean(arr: number[], nullRemove: boolean = true): number {
    if (nullRemove) { // if TRUE, which is the default, then throw out nulls and re-assign to `arr`
        const arr: number[] = arr.filter((elem) => elem !== null);
    }
    // then simply calculate the mean of `arr`
    return arr.reduce((acc, v, i, a) => acc   v / a.length, 0); // https://stackoverflow.com/a/62372003/6105259
}

I then get an error:

Block-scoped variable 'arr' used before its declaration.ts(2448)

I also tried using let in addition or instead of const but it didn't solve the problem.

What am I missing here?

CodePudding user response：

Two options for you:

1. Don't redeclare it, just reassign it:

function calcMean(arr: number[], nullRemove: boolean = true): number {
    if (nullRemove) { // if TRUE, which is the default, then throw out nulls and re-assign to `arr`
        arr = arr.filter((elem) => elem !== null);
        // ^^^ No `const` here
    }
    // then simply calculate the mean of `arr`
    return arr.reduce((acc, v, i, a) => acc   v / a.length, 0); // https://stackoverflow.com/a/62372003/6105259
}

Some folks believe reassigning parameters is poor style (I'm not one of them provided the function is quite small as in your case, but I understand the argument), so alternatively:

2. Assign to a different variable:

function calcMean(arr: number[], nullRemove: boolean = true): number {
    // Remove `null` if requested
    const a = nullRemove ? arr.filter(elem => elem !== null) : arr;
    // then simply calculate the mean of `arr`
    return a.reduce((acc, v, i, a) => acc   v / a.length, 0); // https://stackoverflow.com/a/62372003/6105259
}

CodePudding user response：

The code in your question doesn't currently allow for null values to be included in the number[] parameter.

I like for my code to be explicit about what's happening, so in the case where there might be nulls mixed in the array of numbers, I would explicitly convert them to 0 in the case that they are not removed:

TS Playground

/** Each null element is either omitted or converted to 0 */
function handleNullValues (arr: readonly (number | null)[], omit = true): number[] {
  return omit ?
    arr.filter((value): value is number => value !== null)
    : arr.map(value => value === null ? 0 : value);
}

function calcMean (arr: readonly (number | null)[], nullRemove = true): number {
  const numbers = handleNullValues(arr, nullRemove);
  return numbers.reduce((sum, n) => sum   n) / numbers.length;
}


// Test
console.assert(calcMean([null, 1, 3, 7, 9]) === 5); // ok
console.assert(calcMean([null, 10, 1, 3, 7, 9], false) === 5); // ok