Is there any procedure to check below string as a condition in if statement?

Question

var str='true && (true || false) && true';
if (str) {
    console . log('true');
} else {
    console . log('false');
}

Check above dynamic string as a condition true or false.

An `eval` function exists, but everythinkg which is passed to it will be evaluated, so be carefull with it. It may become a potential risk. — Anthony Raymond, Dec 22 '17 at 13:13
I'd question if this is even needed in the first place. Seems like an odd setup to try and achieve. — Carcigenicate, Dec 22 '17 at 13:14

Christos · Answer 1 · 2017-12-22T13:20:04.993

1

You could make use of eval function. Generally speaking eval evaluates JavaScript code represented as a string. For further info, please have a look here.

However, you should pay attention of the following remark, that you will in the link mentioned above:

eval() is a dangerous function, which executes the code it's passed with the privileges of the caller. If you run eval() with a string that could > be affected by a malicious party, you may end up running malicious code on the > user's machine with the permissions of your webpage / extension.

var str='true && (true || false) && true';
if(eval(str)){
  console.log('true');
}
else{
  console.log('false');
}

edited Dec 22 '17 at 13:20

answered Dec 22 '17 at 13:12

Christos

53,228
8
76
108

1

As a caveat, please refer following before using: [Eval is evil](https://stackoverflow.com/questions/86513/why-is-using-the-javascript-eval-function-a-bad-idea) and [When is JavaScript's eval() not evil?](https://stackoverflow.com/questions/197769/when-is-javascripts-eval-not-evil) and – Rajesh Dec 22 '17 at 13:18
@Rajesh answer restructured. – Christos Dec 22 '17 at 13:20
1

«eval is evil» :) – Aliaksandr Pitkevich Dec 22 '17 at 13:43
1

Thank man this will work for me.this will dangerous but I am not using any authentication process on it. – Rushit Solanki Dec 23 '17 at 03:13

score 0 · Answer 2 · answered Dec 22 '17 at 14:05

You may use a small parser:

 function evaluate(str){
   //remove all whitespace
   str = str.replace(/\s/g,"");
   //Parse stuff in brackets first:
   const first = str.indexOf("("), last = str.lastIndexOf(")");
   if(first != -1 && last != -1)
     str = str.substr(0, first) + evaluate(str.substr(first + 1, last)) + str.substr(last + 1);
  //split into or parts
  return str.split("||")
   //parse the or parts
   .map(sub =>
       //Evaluate the &&
       sub.split("&&")
          .every(el => el === "true")
   //evaluate the or parts
   ).some(e => e);
 }

So you can do:

 if(evaluate("true && false || false")) alert("works!");

Is there any procedure to check below string as a condition in if statement?

2 Answers2