I screw up my coding challenge, but can't find the mistake :(

Just bombed my coding challenge at a company. This was the task: You get a string with operation: 1) If it is a number push it to stack 2) if it is "DUP", then duplicate the one one top and push it to the stack 3) if it is "POP" then remove the one on top 4) if it is "+" add the two numbers on top of the stack and replace both with the result of the addition 5) if it is "-" then minus the two numbers on top (the second minus the first) of the stack and replace both with the result of the subtraction 6) if an error occurs, e.g. not enough numbers to add/subtract/pop, then return -1 Examples: Input: "3 DUP 5 - -" Output: -1 Input: "13 DUP 4 POP 5 DUP + DUP + -" Output: 7 Input: "5 6 + -" Output: -1 My result only got 66% correctness: https://jsbin.com/zifenofoha/1/edit?js,console function solution(S) { // write your code in JavaScript (Node.js 8.9.4) if (!S.length) { // early exit return -1; } const stack = new Array(); const pop = () => { if (!stack.length) { throw new Error('empty'); } return stack.pop(); } const duplicate = () => { try { const dup = pop(); stack.push(dup); stack.push(dup); } catch(e) { throw new Error(e); } }; const add = () => { try { const sum1 = pop(); const sum2 = pop(); stack.push(sum1 + sum2); } catch(e) { throw new Error(e); } }; const minus = () => { try { const min1 = pop(); const min2 = pop(); stack.push(min1 - min2); } catch(e) { throw new Error(e); } }; const ops = { "DUP": duplicate, "POP": pop, "+": add, "-": minus, }; const input = S.split(' '); input.forEach(x => { if (!isNaN(x)) { stack.push(Number(x)); } else { try { if (!ops[x]) { return -1; } ops[x](); } catch(e) { return -1; } } }); try { return pop(); } catch(e) { return -1; } }