Attempting to update a variable within a function results in an error message stating: "Attempting to use block-scoped variable '...' before it has been declared.ts(2448)"

Question

Attempting to update a variable within a function results in an error message stating: "Attempting to use block-scoped variable '...' before it has been declared.ts(2448)"

I'm currently working on a typescript function that takes in a numeric array (defined as type: number[]) and computes the mean. I want to make sure that the function can handle cases where the input array includes some null values. To address this, I introduced an additional parameter that, when set to true, instructs the function to eliminate any nulls before calculating the mean.

However, I'm struggling to find the right approach for this problem since I am unable to modify the original input array within the function.

This is the code snippet for my calcMean() function:

function calcMean(arr: number[], nullRemove: boolean = true): number {
    if (nullRemove) { // If 'true', which is the default value, remove nulls from the array
        const arr: number[] = arr.filter((elem) => elem !== null);
    }
    // Now proceed with calculating the mean of the modified `arr`
    return arr.reduce((acc, v, i, a) => acc + v / a.length, 0); // Reference: https://stackoverflow.com/a/62372003/6105259
}

Upon execution, I encounter the following error message:

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

I attempted using let instead of or alongside const, but unfortunately, it did not resolve the issue.

Can someone guide me on what I might be overlooking here?

typescript function scope declaration

Answer 1

Answer №1

Here are two options for you:

1. Instead of redeclaring it, simply reassign it like this:

function calculateMean(numbers: number[], excludeNulls: boolean = true): number {
    if (excludeNulls) { 
        numbers = numbers.filter((element) => element !== null);
    }
    return numbers.reduce((accumulator, value) => accumulator + value / numbers.length, 0); 
}

Some argue that reassigning parameters is not a good practice, so here's an alternative approach:

2. Assign the filtered array to a different variable:

function calculateMean(numbers: number[], excludeNulls: boolean = true): number {
    const newArray = excludeNulls ? numbers.filter(element => element !== null) : numbers;
    return newArray.reduce((accumulator, value) => accumulator + value / newArray.length, 0);
}

Answer 2

Here are two options for you:

1. Instead of redeclaring it, simply reassign it like this:

function calculateMean(numbers: number[], excludeNulls: boolean = true): number {
    if (excludeNulls) { 
        numbers = numbers.filter((element) => element !== null);
    }
    return numbers.reduce((accumulator, value) => accumulator + value / numbers.length, 0); 
}

Some argue that reassigning parameters is not a good practice, so here's an alternative approach:

2. Assign the filtered array to a different variable:

function calculateMean(numbers: number[], excludeNulls: boolean = true): number {
    const newArray = excludeNulls ? numbers.filter(element => element !== null) : numbers;
    return newArray.reduce((accumulator, value) => accumulator + value / newArray.length, 0);
}

Answer 3

Answer №2

The current code in your question does not handle null values within the number[] parameter.

I prefer my code to be explicit, so if there are any null values mixed in with the array of numbers, I would convert them to 0 explicitly instead of removing them:

/** 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

Edit: Updated based on a comment you made.

TS Playground

function calcMean (arr: readonly (number | null)[]): number {
  let count = 0;
  return arr.filter((value): value is number => {
    const isNumber = typeof value === 'number';
    if (isNumber) count += 1;
    return isNumber;
  }).reduce((sum, n) => sum + n) / count;
}


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

Answer 4

The current code in your question does not handle null values within the number[] parameter.

I prefer my code to be explicit, so if there are any null values mixed in with the array of numbers, I would convert them to 0 explicitly instead of removing them:

/** 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

Edit: Updated based on a comment you made.

TS Playground

function calcMean (arr: readonly (number | null)[]): number {
  let count = 0;
  return arr.filter((value): value is number => {
    const isNumber = typeof value === 'number';
    if (isNumber) count += 1;
    return isNumber;
  }).reduce((sum, n) => sum + n) / count;
}


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

Answer 5

Answer №3

When working with typescript, I encountered the following error message:

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

exportAsExcel () {
   const format = 'xlsx'
   const exportSelectedOnly = true
   const filename = 'test'
   this.$refs.grid.exportTable(format, exportSelectedOnly, filename)
}

To resolve this issue, I made the following changes:

const format = 'xlsx' as string
const exportSelectedOnly = true as boolean
const filename = 'test' as string

Answer 6

When working with typescript, I encountered the following error message:

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

exportAsExcel () {
   const format = 'xlsx'
   const exportSelectedOnly = true
   const filename = 'test'
   this.$refs.grid.exportTable(format, exportSelectedOnly, filename)
}

To resolve this issue, I made the following changes:

const format = 'xlsx' as string
const exportSelectedOnly = true as boolean
const filename = 'test' as string

Attempting to update a variable within a function results in an error message stating: "Attempting to use block-scoped variable '...' before it has been declared.ts(2448)"

Answer №1

Answer №2

Answer №3

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