Find a missing number in a sequence using .reduce

Question

var o = [1,2,3,5,6,7,8]

var res = o.reduce(function(x,y){

   return !((y-x)===1)?y-1:'Nothing'

})
console.log(res)//7

Output should be 4, want to know if it is possible using reduce or functionally (Not through loops)? it only works if the missing value is before the last value of an array.

Answer 1

You can use reduce to compute the actual sum of all elements and then subtract it from the target sum (n(a0+an)/2). This gives you the missing number.

var o = [1,2,3,5,6,7,8];

var len = o.length;
var sum = (len + 1) * (o[0] + o[len - 1]) / 2;
var res = sum  - o.reduce((x,y) => x + y);

console.log(res);

Note that this works with any starting value and any step, e.g. for [3,5,7,11] it will correctly print 9. The only requirement is that o should be an arithmetic progression.