Weblabor

function Calculator(result) {
  this.result = result && parseInt(result) || 0;
}

Calculator.prototype = {
  // Equality operator. Sets the result to the current value.
  '=': function (value) {
    this.result = value;
    return this;
  },
  // Addition operator. Adds the value to the actual result.
  '+': function (value) {
    this.result += value;
    return this;
  },
  // Negation operator. Substracts the value from the actual result.
  '-': function (value) {
    this.result -= value;
    return this;
  },
  // Multiplication operator. Multiplies the actual result with value.
  '*': function (value) {
    this.result *= value;
    return this;
  },
  // Division operator. Divides the actual result by the value.
  '/': function (value) {
    this.result /= value;
    return this;
  },
  // Matches a math operation and a number
  parser: /(^=|[+\-\/*])([0-9]*\.?[0-9]+([eE][-+]?[0-9]+)?)/g,
  // Parse an expression that is matched by the parser.
  parse: function (text) {
    var match;
    while ((match = this.parser.exec(text))) {
      this[match[1]](parseFloat(match[2]))
    }
    return this;
  },
  // Return the current result of the calculation.
  valueOf: function () {
    return this.result;
  }
};

var text = "=8+2-1", calc;
calc = new Calculator();
console.log(+calc.parse(text));