modd-coding-challenge.js

const input = [
  "AND",
  ["<", "var1", "var2"],
  [
    "OR",
    [">", "var3", "var4"],
    ["==", "var5", "var6"]
  ]
];
const result = "var1 < val2 AND (var3 > val4 OR val5 == val6)";

function stringify(array, depth) {
  let left = '';
  let right = '';
  const operator = array[0];

  if (operator === "AND" || operator === "OR") {
    left = stringify(array[1], 1);
    right = stringify(array[2], 1);
    if (depth) {
      left = '(' + left;
      right += ')';
    }
  } else {
    left = array[1];
    right = array[2];
  }
  return `${left} ${operator} ${right}`;
}

function changeNames(input) {
  return input.replace(/var[2,4-6]/g, (match) => {
    return match.replace('r', 'l');
  })
}

const output = changeNames(stringify(input));

console.log(output);
console.log(output === result);