javascript conditional expression parser

Question

I am working in JavaScript. I want to parse and evaluate conditional expression. ex:

    var param1=1;
    var param2=2;
    var param3=3;
    var expression="(param1==param2)||(param3<param1)";

I want to write a function which will accept 'expression' as a input and parse the expression as well as evaluate expression and return evaluated result.

Please let me know for any suggestions.

Thanks in advance.

Answer 1

Here is the evil one: eval();

var param1=1;
var param2=2;
var param3=3;
var expression=eval("(param1==param2)||(param3<param1)");

Then your function comes,

function myEvaluator(s) {
    return eval(s);
}

You must have variables in expression public.

Answer 2

This is a really old question and I am surprised that no libraries came out since to help with this.

Out of necessity, I built a library called angel-eval that does exactly that. It parses a string expression and evaluates with a context object. It uses a parser generator called Nearley under the hood and does not use eval or new Function(…).

To use, install: npm i angel-eval

const { evaluate } = require("angel-eval");

evaluate("(param1===param2)||(param3<param1)", { param1: 1, param2: 2, param3: 3 });

angel-eval does not support == or != so you need to use === and !==.

Here is a sandbox:

Answer 3

There are a few expression parsers out there, but I would like to suggest mine. It's called swan-js and it doesn't use eval or new Function.

Installation:

npm install @onlabsorg/swan-js

Usage (it works both in NodeJS and in the browser):

const swan = require('@onlabsorg/swan-js');
const evaluate = swan.parse("param1 == param2 | param3 < param1");
const context = swan.createContext({param1:1, param2:2, param3:3});
const value = await evaluate(context); // false