calculator.js

calculator.js

/* input is string (with spaces) as below expressions.
  operator precedes numbers
  "+ 4 5" --> result: 9,
  "+ + 3 4 7" --> 14,
  "- 3 * 4 5" --> -17,
  "* 3 + 4 5" --> 27,
  "* + 3 4 5" --> 35,
  "+ - 3 4 7" --> 6,
  "+ - * 3 2 4 5" --> 7,
  "+ 24 * - 2 1 1" --> 25
*/

function calculate (expression) {
  return calc(expression.split(" "))
}

function calc (expression) {
  const current = expression[0]
  const left = expression.slice(1, -1)
  const leftVal = left[0]
  const right = expression.slice(2)
  const rightVal = expression[expression.length - 1]

  if (expression.length === 1 && RegExp(/\d/).test(expression)) {
    return Number(current)
  }
  
  switch (current) {
    case '+':
      return RegExp(/\d/).test(leftVal) 
        ? (Number(leftVal) + (expression.length === 3 ? Number(rightVal) : calc(right))) 
        : (calc(left) + Number(rightVal))
    
    case '-':
      return RegExp(/\d/).test(leftVal) 
        ? (Number(leftVal) - (expression.length === 3 ? Number(rightVal) : calc(right))) 
        : (calc(left) - Number(rightVal))
    
    case '*':
      return RegExp(/\d/).test(leftVal) 
        ? (Number(leftVal) * (expression.length === 3 ? Number(rightVal) : calc(right))) 
        : (calc(left) * Number(rightVal))
    
    case '/':
      return RegExp(/\d/).test(leftVal) 
        ? (Number(leftVal) / (expression.length === 3 ? Number(rightVal) : calc(right))) 
        : (calc(left) / Number(rightVal))

    default:
        console.log('nothing got into the default case')
  }
}