var cellValue1 = '= a2 + 1 - a1 + average(a1, a2, a3) + sum(a2, a3)'
var cellValue2 = '= sum(10, 20)'
var cellValue3 = '100'

console.log(computeCell(cellValue1))
console.log(computeCell(cellValue2))
console.log(computeCell(cellValue3))

function computeCell(value) {
  if (!isExpression(value)) {
    return value
  }
  return computeExpression(value)
}

function computeExpression(expression) {
  var expressionReplaced = expression.toUpperCase()
    // Strip all spaces
    .replace(/\s/g, '')

    // Strip "=" at beggining of expression
    .replace(/^\s*=\s*/, '')

    // Replace cell references with their values
    .replace(/[A-Z]+\d+/g, function (match) {
      return getCellValue(match)
    })

    // Replace SUM expressions with their results
    .replace(/SUM\((.*?)\)/g, function (sumExpression, sumExpressionInner) {
      var sumValues = sumExpressionInner.split(',').map(Number)
      return sumValues.reduce(function (sum, num) {
        return sum + num
      }, 0)
    })

    // Replace AVERAGE expressions with their results
    .replace(/AVERAGE\((.*?)\)/g, function (avgExpression, avgExpressionInner) {
      var avgValues = avgExpressionInner.split(',').map(Number)
      return avgValues.reduce(function (avg, num) {
        return avg + num
      }, 0) / avgValues.length
    })
  return evalMath(expressionReplaced)
}

function evalMath(expression) {
  return eval(expression)
}

function getCellValue(coords) {
  var values = {
    A1: 5.5,
    A2: 10,
    A3: 6.6,
  }
  return values[coords]
}

function isExpression(value) {
  return /^\s*=\s*/.test(value)
}