Parsing & Checking Condition Expression in Javascript

Question

I have string that have variable conditions in it, like this:

(@IS_VERIFIED = 'True' OR @CONFIRMATION_NEEDED != 'True') AND @REQUEST_LIMIT != '0'

This is just an example and there are unknown number of variables & cases.

Every variable starts with @
There are ORs and ANDs with sometimes parenthesis
Values are always in quotation marks like 'xyz', so all can be considered strings.
Conditions are always checked with either = or !=

I also have a javascript map which holds all the variables like:

const vars = {
 IS_VERIFIED: 'True',
 CONFIRMATION_NEEDED: 'True',
 REQUEST_LIMIT: '2'
}

What I need to do is parse the condition string and check the values using the map above to reach a final result, true or false. I came up with few ideas but they were all too complicated. Is there any known, easy method for this or do I have to create something from scratch?

Any help is appreciated, thanks.

Edit: After achieving this, next goal for me will be showing which variables break the condition, somehow.

Regular Expressions might come in handy here. Did you try using that? — Anindya Dey
– Anindya Dey, Commented Apr 21, 2022 at 10:30
@AnindyaDey yeah, I thought about it but couldn't figure out how to apply it to cases where there might be multiple paranthesis inside each other and all. I think I have to create a recursive logic but not sure how. — Nanoturka
– Nanoturka, Commented Apr 21, 2022 at 10:32
I can think of a solution using eval, but are you willing to use eval? — Som Shekhar Mukherjee
– Som Shekhar Mukherjee, Commented Apr 21, 2022 at 11:03

Som Shekhar Mukherjee · Accepted Answer · 2022-04-21 11:10:22Z

1

Caution: `eval` solution ahead, be very careful while using this!

Simply modify the string to be a valid JS expression and then use eval.

const vars = {
  IS_VERIFIED: "True",
  CONFIRMATION_NEEDED: "True",
  REQUEST_LIMIT: "2",
};

const str = `(@IS_VERIFIED = 'True' OR @CONFIRMATION_NEEDED != 'True') AND @REQUEST_LIMIT != '0'`;

const evalStr = str
  .replaceAll("@", "vars.")
  .replaceAll("OR", "||")
  .replaceAll("AND", "&&")
  .replaceAll("!=", "!")
  .replaceAll("=", "===")
  .replaceAll("!", "!==");

const res = eval(evalStr);
console.log(res);

answered Apr 21, 2022 at 11:10

Som Shekhar Mukherjee

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3 Comments

Aioros Over a year ago

I know eval is not going to be liked much here, but I loved this. Very clever.

Nanoturka Over a year ago

I thought about using eval but the condition string will be a user input, it is not an option for me due to security risks and I am not allowed to use it. Also even if I could eliminate the security risks somehow, than it would be impossible to do the next step(check the edit). So I guess I have to parse the string somehow.

Som Shekhar Mukherjee Over a year ago

@Nanoturka That's unfortunate! Some fancy string parsing with regex might be able to help.

Anindya Dey · Accepted Answer · 2022-04-21 11:21:05Z

0

Here is a snippet using RegExp:

let test = "@XYZ='True' AND @ABC='False'";

const regexp = /(@\w+)|(\'\w+\')/g;

const regexpResult = [...test.match(regexp)]
// Returns something like this:
// ["@XYZ", "'True'", "@ABC", "'False'"]

const vars = {};

for (let i = 0; i < regexpResult.length; i += 2) // We will be using two consecutive values at a time
{
  let keyName = regexpResult[i].replace("@", "");
  let keyValue = regexpResult[i+1].replaceAll("'", "");
  
  vars[keyName] = keyValue;
}

// OUTPUT of "vars":
// {
//   XYZ: "True",
//   ABC: "False"
// }

I hope this gives you an idea about how to solve your problem.

answered Apr 21, 2022 at 11:21

Anindya Dey

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