Removing the second number of the pair that add up to a target in JavaScript

Problem

We need to write a JavaScript function that takes an array of numbers and a target sum. The function should remove the second number from any consecutive pair of numbers that add up to the target.

Example

Let's see how this works with a practical example:

const arr = [1, 2, 3, 4, 5];
const target = 3;

const removeSecond = (arr = [], target = 1) => {
    const res = [arr[0]];
    for(let i = 1; i < arr.length; i++){
        if(arr[i] + res[res.length-1] !== target){
            res.push(arr[i]);
        }
    }
    return res;
};

console.log(removeSecond(arr, target));

[ 1, 3, 4, 5 ]

How It Works

The algorithm works by:

• Starting with the first element in the result array
• Checking each subsequent element against the last element in our result
• If they don't sum to the target, adding the current element
• If they do sum to the target, skipping the current element (removing the second number)

Step-by-Step Breakdown

const arr = [1, 2, 3, 4, 5];
const target = 3;

const removeSecondDetailed = (arr, target) => {
    const res = [arr[0]]; // Start with first element: [1]
    console.log("Initial result:", res);
    
    for(let i = 1; i < arr.length; i++){
        const currentSum = arr[i] + res[res.length-1];
        console.log(`Checking ${arr[i]} + ${res[res.length-1]} = ${currentSum}`);
        
        if(currentSum !== target){
            res.push(arr[i]);
            console.log("Added to result:", res);
        } else {
            console.log(`Sum equals target (${target}), skipping ${arr[i]}`);
        }
    }
    return res;
};

console.log("Final result:", removeSecondDetailed(arr, target));

Initial result: [ 1 ]
Checking 2 + 1 = 3
Sum equals target (3), skipping 2
Checking 3 + 1 = 4
Added to result: [ 1, 3 ]
Checking 4 + 3 = 7
Added to result: [ 1, 3, 4 ]
Checking 5 + 4 = 9
Added to result: [ 1, 3, 4, 5 ]
Final result: [ 1, 3, 4, 5 ]

Another Example

const arr2 = [2, 4, 6, 1, 5];
const target2 = 6;

console.log("Original array:", arr2);
console.log("Target sum:", target2);
console.log("Result:", removeSecond(arr2, target2));

Original array: [ 2, 4, 6, 1, 5 ]
Target sum: 6
Result: [ 2, 6, 1, 5 ]

Conclusion

This algorithm efficiently removes the second number from consecutive pairs that sum to a target value. It maintains the original order while processing elements sequentially, making it a simple yet effective solution for this specific array manipulation problem.