functional style JS

gistfile1.txt

let existing = "(((4 + 3) * 2) + 9)";
const operators = ['*', '/', '+', '-'];

// find positions of operators in "existing"
let positions = existing
                  .split("")
                  .reduce((acc, character, idx) => {
                    if (operators.includes(character))
                      acc.push(idx)
                    return acc;
                  }, []);

// outputs: [5, 10, 15]
console.log(positions);