Question

I have a script which takes user input, performs some operations on that input and then executes the modified input.

Example:

User enters ‘vbscipt or javascript’

My script converts it to:

var searchtest = "mystring.search('vbscript')>=0 || mystring.search('javascript')>=0";

And executes it using eval(searchtest);

However if the user enters ‘vbscript javascript’

My script converts it to

var searchtest = "mystring.search('vbscript')>=0 mystring.search('javascript')>=0";

(Note lack of ||) Which will cause an error when I call eval(searchtest);

Is there a way to test the searchtest string to determine if it is a valid javascript expression before executing it?

Answer 1

JavaScript eval is evil !!

Instead of transforming user input as you are doing why not something like the following :

// list of values considered valid
myString = ['javascript', 'vbscript', 'perl', 'python'];

function search (x) {
  // Returns true if x is a value an array (passed as `this`)
  return this.indexOf (x) >= 0;
}

function searchTest (userInput, myString, search) { 'use strict';

  function orClause (clause) {
    // caluse is a series of values separated by `or`, return true
    // if any search returns true for any of the values.
    clause = clause.split (/\s+or\s+/i); // split on 'or'
    while (clause.length) { // for each value
      if (search.apply (myString, [clause.shift ()])) {
        return true;
      }
    }
    return false; // no matches....
  }        

  userInput = userInput.split (/\s+and\s+/i); // split on 'and'

  // userInput is now an array of clauses each is either a value or 
  // a series of values separated by `or`. We pass each clause to
  // the orCaluse function and return true only if ALL returns are true. 
  while (userInput.length) { 
    if (!orClause (userInput.shift ())) { 
       return false; // and fails
    }
  }

  return true;  // All true so return true.
}

searchTest ('fred or javascript and python', myString, search),

I am assuming your expresson can be a sequence of ands and ors with ands taking precedence