Adding only odd or even numbers JavaScript

We are required to make a function that given an array of numbers and a string that can take any of the two values "odd" or "even", adds the numbers which match that condition. If no values match the condition, 0 should be returned.

For example:

conditionalSum([1, 2, 3, 4, 5], "even") => 6
conditionalSum([1, 2, 3, 4, 5], "odd") => 9
conditionalSum([13, 88, 12, 44, 99], "even") => 144
conditionalSum([], "odd") => 0

Using Array.reduce() Method

We'll use the Array.prototype.reduce() method to iterate through the array and sum numbers based on the condition:

const conditionalSum = (arr, condition) => {
    const add = (num1, num2) => {
        if(condition === 'even' && num2 % 2 === 0){
            return num1 + num2;
        }
        if(condition === 'odd' && num2 % 2 === 1){
            return num1 + num2;
        };
        return num1;
    }
    return arr.reduce((acc, val) => add(acc, val), 0);
}

console.log(conditionalSum([1, 2, 3, 4, 5], "even"));
console.log(conditionalSum([1, 2, 3, 4, 5], "odd"));
console.log(conditionalSum([13, 88, 12, 44, 99], "even"));
console.log(conditionalSum([], "odd"));

6
9
144
0

Alternative Approach Using Filter and Reduce

Here's a cleaner approach that first filters the array based on the condition, then sums the filtered values:

const conditionalSumFiltered = (arr, condition) => {
    return arr
        .filter(num => condition === 'even' ? num % 2 === 0 : num % 2 === 1)
        .reduce((sum, num) => sum + num, 0);
}

console.log(conditionalSumFiltered([1, 2, 3, 4, 5], "even"));
console.log(conditionalSumFiltered([1, 2, 3, 4, 5], "odd"));
console.log(conditionalSumFiltered([13, 88, 12, 44, 99], "even"));
console.log(conditionalSumFiltered([], "odd"));

6
9
144
0

How It Works

Both approaches work by:

• Checking conditions: Using modulo operator (%) to determine if a number is odd (num % 2 === 1) or even (num % 2 === 0)
• Accumulating sum: Adding numbers that match the specified condition
• Handling empty arrays: Returns 0 when no elements match the condition

Conclusion

The filter-reduce approach is more readable and follows functional programming principles. Both methods effectively sum odd or even numbers from an array based on the provided condition.