i am trying to convert a string value to expression so that i can use it in if condition like:

var StringVal = '20 > 18 && "yes" == "yes"';

if(StringVal){

    ....
}

is it possible to do this, please suggest me.

Thanks

i am trying to convert a string value to expression so that i can use it in if condition like:

var StringVal = '20 > 18 && "yes" == "yes"';

if(StringVal){

    ....
}

is it possible to do this, please suggest me.

Thanks

• javascript

• eval would do it, or most likely the Function equivalent. – Xotic750 Commented Sep 10, 2016 at 13:04
• yes..but it will not work for "yes" == "yes" – Rahul Sharma Commented Sep 10, 2016 at 13:05
• 1 Why do you need to do this? – Xotic750 Commented Sep 10, 2016 at 13:06
• i am working on some survey app and from web service i am getting: [[abc]] > 18 && [[xyz]] == "yes" [[abc]] and [[xyz]] stands from previous questions answer and i just replace that [[..]] string to real values and after that i have to check the condition.. – Rahul Sharma Commented Sep 10, 2016 at 13:10
• i think eval is working... – Rahul Sharma Commented Sep 10, 2016 at 13:10

3 Answers 3

It's not generally safe to take input from a user source and evaluate it, but you could use Function evaluation, or eval

var StringVal = '20 > 18 && "yes" == "yes"';
if (new Function('return (' + StringVal + ')')()) {
  console.log('ok');
}

Are eval() and new Function() the same thing?

Update: If you could give further information in your question as to why you feel this is necessary along with an example of your actual code, then a safer solution could be suggested.

Further: In JSON2, eval is used in JSON.parse

https://github./douglascrockford/JSON-js/blob/master/json2.js#L491

but you will also notice that some sanitising is performed before eval is called.

You can use function eval()

try this:

var StringVal = '20 > 18 && "yes" == "yes"';

if(eval(StringVal)){

    ....
}

other example:

var a = '1==3'
var b = '1==1'

console.log(eval(a));
// false
console.log(eval(b));
// true

Note that eval won't directly parse your text as an expression, e.g.

eval('{}'); // undefined

That's because {} is parsed as a block, not as an object initializer expression.

There are several ways to force it to be parsed as an expression, like nesting inside parentheses, or via a return statement. So I would use

function evalExpr(expr) {
  Function("return " + expr + ";");
  return eval("(" + expr + ")");
}
console.log(evalExpr('{}')); // Object {}
try { evalExpr('1),(2') } catch(err) { console.error(err); } // SyntaxError
try { evalExpr('1;2') } catch(err) { console.error(err); } // SyntaxError

eval("(" + expr + ")") evaluates as an expression, and previously Function("return " + expr + ";") checks that the expression does not contain an invalid ) to escape the wrapping parentheses.