rpn/index.js

Reverse Polish Notation implementation.

'use strict';

/**
 * @param {string} postfix
 * @returns {number}
 */
function rpn(postfix) {
  if (postfix.length === 0) {
    return 0;
  }

  // Split into array of tokens
  postfix = postfix.split(/\s+/);

  /** @type {number[]} */
  const stack = [];

  for (let i = 0; i < postfix.length; i++) {
    const token = postfix[i];

    // Token is a value, push it onto the stack
    if (!Number.isNaN(+token)) {
      stack.push(parseFloat(token));
    }

    // Token is operator
    else {
      // Every operation requires two arguments
      if (stack.length < 2) {
        throw new Error('Insufficient values in expression.');
      }

      // Pop two items from the top of the stack and push the result of the
      // operation onto the stack.
      const y = stack.pop();
      const x = stack.pop();
      /* eslint no-eval: 0 */
      stack.push(eval(x + token + ' ' + y));
    }
  }

  if (stack.length > 1) {
    throw new Error('Inputted expression has too many values.');
  }

  return stack.pop();
}

module.exports = rpn;