Parse complex String Expression to Boolean in Javascript

Question

I've got the Task to write a filter for complex and nested JSONs. To do this I need to evaluate Boolean Expressions which the user enters in an input field. For the sake of the question lets say we have got the following input:

(Key1 = "foo" || key2 = 2 && key3 = "bar") && key4 = 7

I managed to write a function to evaluate each keyValue-Pair to a either "True" or "False" and replaced them in the string: e.g.:

(True || False && True) && True

Now I want to parse this String to a boolean Expression. Is there any "easy" way (built in Function, Plugin) for Javascript which can parse this String?

of course its && thanks for the hint! I changed it in the question — GuyWithCookies, Aug 16 '16 at 08:35
It is never a Boolean Expression I just created a regexp to get the keyVal Pairs and search them in the JSON. The function returns true or false and I wrote it back in the String. This was just my current approach. I am open to any better idea! — GuyWithCookies, Aug 16 '16 at 08:40

martin · Accepted Answer · 2016-08-16T08:37:41.973

There's eval() which is usually not recommended to use but maybe in your case it's suitable. See for more details: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval

You could write:

eval("(true || false && true) && true") // Returns: 1

Note, that I had to rewrite True to true because True is not a recognised as a boolean expression.

However, scopes are applied to eval() so I could also write:

var True = 1;
var False = 0;
eval("(True || False && True) && True"); // Returns: 1

This could work for you if you know the variables in advance I guess. Otherwise, maybe have a look at https://silentmatt.com/javascript-expression-evaluator/ or http://jsep.from.so/.

milahu · Answer 2 · 2020-11-22T19:03:12.460

https://github.com/joewalnes/filtrex

A simple, safe, JavaScript Filter Expression compiler for end-users

// Input from user (e.g. search filter)
var expression = '(key1 == "foo" || key2 == 2 && key3 == "bar") && key4 == 7';

// Compile expression to executable function
var myfilter = compileExpression(expression);

// Execute function
myfilter({Key1: "foo", key2: 2, key3: "bar", key4: 7}); // returns 1
myfilter({}); // returns 0

// filter an array of objects
var data = [{key1: "foo"}, {key1: "foo2"}];
var results = data.filter(myfilter);

the cost of this beauty: 92KByte minified bundle size

i started the project logic_fn, to do a similar job in a 1KByte minified bundle, by translating a custom logic expression (with custom operators like &, AND, -) to a javascript expression string, and feeding that to var f = new Function('input', 'return ' + expression + ';'). new Function is a bit more secure than eval, but i still must defend against arbitrary code execution attacks.

score 1 · Answer 3 · answered Jan 12 '19 at 15:31

To avoid using eval, I used http://jsep.from.so/ and then wrote a function to evaluate the output from jsep:

function evaluateBooleanExpressionTree(tree) {
    if (tree.type == "Literal") {
        return tree.value;
    }
    else if (tree.type == "LogicalExpression") {
        if (tree.operator == "&&") {
            return evaluateBooleanExpressionTree(tree["left"]) && this.evaluateBooleanExpressionTree(tree["right"]);
        }
        else {
            return evaluateBooleanExpressionTree(tree["left"]) || this.evaluateBooleanExpressionTree(tree["right"]);
        }
    }
    else {
        // Unexpected node parsed
        return null;
    }
}

evaluateBooleanExpressionTree(jsep("(true || false && true) && true"))
// true

working link: https://github.com/EricSmekens/jsep - problem is, jsep does not generate a filter function, that can directly reduce the input data to true or false — milahu, Nov 22 '20 at 18:40

score 1 · Answer 4 · answered Mar 29 '21 at 09:04

Never use eval()!

I also wrote a small expression parser for my projects. It's missing a few operators and features but is pretty useful, and super light (1.6k minified)

Here's how it works :

input an object "subject", and a logical expression string
find deepest nested parentheses group
find operators
process group and replace output in top level group
loop

https://github.com/nicopowa/operators

var myObject = {key1: "foo", key2: 2, key3: "bar", key4: 7};
var myExpression = "(key1 || key2 && key3) && key4";
var result = Operate.parse(myObject, myExpression);

score 0 · Answer 5 · edited May 23 '17 at 12:00

0

It seems like it's a perfect place to use eval. However, look through answers to this SO question to learn about drawbacks of this solution.

eval reference @ w3schools

edited May 23 '17 at 12:00

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answered Aug 16 '16 at 08:33

Krzysztof Zbiciński

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Parse complex String Expression to Boolean in Javascript

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