Calculate string value in javascript, not using eval

There's nothing wrong with eval, especially for cases like this. You can sanitize the string with a regex first to be safe:

// strip anything other than digits, (), -+/* and .
var str = "12/5*9+9.4*2".replace(/[^-()\d/*+.]/g, '');
alert(eval(str));

Eval was built for conditions like this.

If you wanted another method, you'd have to use a pure Javascript implementation of the exact thing eval is going to do.

  • The hard part is not the parsing of numbers and operators
  • The hard part is applying order of operation and recursive control

Here's a quick basic example I came up with (updated (2011-06-26): cleaner w/ input boxes).
http://jsfiddle.net/vol7ron/6cdfA/

Note:

  • it only handles the basic operators
  • it does not check the validity of the numbers (example: divide by zero)
  • it has not implemented parenthetical operation
  • for all these reasons and more, eval would be a better choice

Edit (2017-05-26) to use SO Snippet:

function calculate(input) {

  var f = {
    add: '+',
    sub: '-',
    div: '/',
    mlt: '*',
    mod: '%',
    exp: '^'
  };

  // Create array for Order of Operation and precedence
  f.ooo = [
    [
      [f.mlt],
      [f.div],
      [f.mod],
      [f.exp]
    ],
    [
      [f.add],
      [f.sub]
    ]
  ];

  input = input.replace(/[^0-9%^*\/()\-+.]/g, ''); // clean up unnecessary characters

  var output;
  for (var i = 0, n = f.ooo.length; i < n; i++) {

    // Regular Expression to look for operators between floating numbers or integers
    var re = new RegExp('(\\d+\\.?\\d*)([\\' + f.ooo[i].join('\\') + '])(\\d+\\.?\\d*)');
    re.lastIndex = 0; // take precautions and reset re starting pos

    // Loop while there is still calculation for level of precedence
    while (re.test(input)) {
      output = _calculate(RegExp.$1, RegExp.$2, RegExp.$3);
      if (isNaN(output) || !isFinite(output)) 
        return output; // exit early if not a number
      input = input.replace(re, output);
    }
  }

  return output;

  function _calculate(a, op, b) {
    a = a * 1;
    b = b * 1;
    switch (op) {
      case f.add:
        return a + b;
        break;
      case f.sub:
        return a - b;
        break;
      case f.div:
        return a / b;
        break;
      case f.mlt:
        return a * b;
        break;
      case f.mod:
        return a % b;
        break;
      case f.exp:
        return Math.pow(a, b);
        break;
      default:
        null;
    }
  }
}
label {
  display: inline-block;
  width: 4em;
}
<div>
  <label for="input">Equation: </label>
  <input type="text" id="input" value="12/5*9+9.4*2-1" />
  <input type="button" 
         value="calculate" 
         onclick="getElementById('result').value = calculate(getElementById('input').value)" />
</div>

<div>
  <label for="result">Result: </label>
  <input type="text" id="result" />
</div>

This exactly the place where you should be using eval, or you will have to loop through the string and generate the numbers. You will have to use isNaN method to do it.


Mhh, you could use the Function-constructor:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function

function evil(fn) {
  return new Function('return ' + fn)();
}

console.log( evil('12/5*9+9.4*2') ); // => 40.4

Tags:

Javascript

Eval

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