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
3 Answers
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.com/douglascrockford/JSON-js/blob/master/json2.js#L491
but you will also notice that some sanitising is performed before eval is called.
5 Comments
spongessuck
Don't use this method! It is dangerous and unnecessary.
Xotic750
This answers the OP question and states that it is generally not a safe way to do it, I feel the down-vote is unnecessary.
spongessuck
It is because the OP shouldn't do it this way. The OP doesn't know the correct way do to it and you're encouraging bad practice.
Xotic750
No, I'm not encouraging bad practice.
Oriol
Be aware this won't recognize syntax errors like
'true);(0'.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
Comments
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); } // SyntaxErroreval("(" + expr + ")") evaluates as an expression, and previously Function("return " + expr + ";") checks that the expression does not contain an invalid ) to escape the wrapping parentheses.
evalwould do it, or most likely theFunctionequivalent."yes" == "yes"[[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..