分かったよじゃあoverlappingPairsCollect実装するよ…

const overlappingPairsCollect = (arr, cb) => {
let head = arr.shift();
const overlappingPairs = [];
for (const value of arr) {
overlappingPairs.push([head, value]);
head = value;
}
return overlappingPairs.map(([a, b]) => cb(a, b));
}

overlappingPairsCollect([3,1,4,1,5,9,2,6,5], (a, b) => b - a);
//=> [-2,3,-3,4,4,-7,4,-1]