You could use reduce method and check if current element is < 0 and greater then accumulator value.
var arr = [-7, -5, -4, -2, 1, 3, 9];
const result = arr.reduce((r, e) => {
if(e < 0 && r < e) r = e;
return r;
})
console.log(result)To get index you could use one object as {value, index} as accumulator and get index.
var arr = [-7, -5, -4, -2, 1, 3, 9];
const result = arr.reduce((r, val, index) => {
if(index == 0 || val < 0 && r.val < val) r = {val, index}
return r;
}, 0).index
console.log(result)