Array Merge Util (takes an ordered array and creates a new array with the value that overlap included while the values outside are left alone)

merge.js

const src = [3, 4, 8, 9]

/**
 * @param {number[]} input 
 * @param {number[]} source 
 * @returns 
 */
function update(input, source) {
    if (input.length === 0) return [...source]
    if (source.length === 0) return [...input]

    let first = input[0]
    let last = input[input.length - 1]

    let leading_index = 0
    for (; first >= source[leading_index] && leading_index < source.length; leading_index++) { }

    let trailing_index = source.length - 1
    for (; last < source[trailing_index] && trailing_index >= leading_index; trailing_index--) { }

    return [...source.slice(0, leading_index), ...input, ...source.slice(trailing_index + 1)]
}

//    [3, 4,   8, 9]
// [2, 3,    5]
console.log(update([2, 3, 5], src)) // [2, 3, 5, 8, 9]

//    [3, 4,   8, 9]
// [2, 3,    5,     10]
console.log(update([2, 3, 5, 10], src)) // [2, 3, 5, 10]

//       [3, 4, 8, 9]
// [1, 2]
console.log(update([1, 2], src)) // [1, 2, 3, 4, 8, 9]

// [3, 4, 8, 9]
//            [10, 11]
console.log(update([10, 11], src)) // [3, 4, 8, 9, 10, 11]

// [3, 4,        8, 9]
//      [5, 6, 7]
console.log(update([5, 6, 7], src)) // [3, 4, 5, 6, 7, 8, 9, 10, 11]

console.log(update([1, 2, 3], [])) // [1, 2, 3]

console.log(update([], [3, 4, 5])) // [3, 4, 5]