[1,7][2,6][3,5][4,4]

var a= [1,2,3,4,5,6,7]
let sum = function(a) {
for ( i=0; i c a.length;i++)
{
for( j= a.length-1;j>=i;j--)
{
if( a[i] + a[j] == 8 )
console.log([a[i],a[j]])
}
}}
console.log(sum(a));