get deriv of function

index.js

const readline = require("readline-sync");

let equation = readline.question("equation >  ");
equation = equation.split(/ /g);
let eq = "";
for (const term of equation) {
  if (term.match(/(\+|-)/g)) {
    eq += term;
    continue;
  }
  let n = term.substr(term.indexOf("^") + 1, term.length);
  let k = term.substr(0, term.indexOf("x")) || 1;
  console.log(deriv(k, n));
  eq += deriv(k, n);
}
console.log(eq);

/* let k = readline.question("k >  ", { hideEchoBack: false });

let n = readline.question("n >  ", { hideEchoBack: false });

console.log(`${k * n}x${n - 1 === 0 ? "^" + n - 1 : ""}`);
 */

function deriv(k, n) {
  return `${k * n}x${n - 1 !== 0 ? "^" + (n - 1).toString() : ""}`;
}