How do I convert string math expression to an actual math expression

Question

I have a string expression like this 10/3*4-7 and I want to know if there is some type of javascript method that can convert the string into an actual expression so that I can get the answer . I have a function that randomly generates numbers and operators, and I want to get the result from the equation. However, the function that I created to get the result only does the calculation from left to right. It does not go by a operator priority, where * and / get calculated first and stuff like that. Is there something in javascript that can do that or would I have to create a function to do something like that?

Answer 1

I'd recommend using a library such as Math.js as this will parse the math expression using a regular expression. This will avoid opening the application up to Cross-site scripting (XSS) attacks.

var answer = math.eval('10/3*4-7');
console.log(answer);

<script src="https://cdnjs.cloudflare.com/ajax/libs/mathjs/3.6.0/math.min.js"></script>

Answer 2

This function takes either a string expression, or an array of characters that create an expression.

function calculate(expression) {
  "strict mode";
 if(Array.isArray(expression))
  expression = expression.join("");
  var allowedChars = "1234567890%^*()-+/. ";
  for (var i = 0; i < expression.length; i++)
    if (allowedChars.indexOf(expression.charAt(i)) < 0)
      throw new Error("Invalid expression: unexpected '" + expression.charAt(i) + "' in '"+expression+"'");
  return eval(expression);
}

try{
 alert(calculate("4+4"));
 alert(calculate([7,"+",9,"/",34]));
 alert(calculate("45/3"));
 alert(calculate("100 * cheeseburger"));
}catch(e){
 alert(e.message);
}

Here's why this is safe:

1. Improper use of eval opens up your code for injection attacks
   • In strict mode eval is not able to introduce new variables to the surrounding scope. (As it's at the end of it's own function, nothing else is happening in that scope anyway.)
   • The function will throw an error if any character is present which is not part of a mathematical expression, making injection impossible.
2. With eval, debugging can be more challenging (no line numbers, etc.)
   • In this case the function throws an Error, which includes a line number. The eval'd code is only so a single line, so this is not relevant.
3. eval'd code executes more slowly (no opportunity to compile/cache eval'd code)
   • In this case there is nothing to compile/cache, so this is not an issue.

Answer 3

There is quite a discussion here on why you should not use eval() unless there are no clear alternatives. Using eval() in this case is completely unnecessary as shown in the example below:

var expression = "10/3*4-7";
var foo = Function('"use strict"; return '+expression);

console.log(foo());