How can I return true or false based on a string based expression in JavaScript?

Question

I'm needing to create a dynamic validator based on a string value that's passed into a json object. So let's say you have the following string:

var required: "Homes > 0"

In this example, "Homes" is an accessible object within my function. I want to logically turn the above into:

if (this.Homes > 0) { return true; }

Thinking about this in parts:

if (this[left] > parsed[right]) { return true; }

I think you get the idea. I'm not sure if there's a way to easily extract operators without just doing a switch for every type? As in:

required: "Homes = 0" // this[Left] === 0

I'm about to do this in a very horrifying ugly string splitting way with a switch case on the operators. Was just wondering if there was a super slick way to make something like this robust.

What if instead of a string you accept a function: var required = function(o) { return o.Homes > 0; };? — zerkms
– zerkms, Commented Aug 28, 2017 at 21:41
Hmm. Well I'm going to try that right now. My initial goal was to make it human friendly in a string format. — user1447679
– user1447679, Commented Aug 28, 2017 at 21:43
You either have to write your own parser, or use someone else's. eval and the Function constructor give you access to the built–in parser. You can also insert code as the content of a script node. Take your pick. — RobG
– RobG, Commented Aug 28, 2017 at 23:41

HumbleWebDev · Accepted Answer · 2017-08-28 21:43:36Z

1

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval

eval('Homes > 0');

but please consider using this pattern instead:

function Class(){
     this.validate = function(){
         return true;
     }
}

var x = new Class();
//overload the validate operator
x.validate = function(){
    return false;
}

You want to avoid eval because it is potentially unsafe because it allows for arbitrary code execution.

answered Aug 28, 2017 at 21:43

HumbleWebDev

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Comments

Mario Lopes · Accepted Answer · 2017-08-28 21:44:44Z

0

Try using

eval("if (this.Homes > 0) { return true; }");

answered Aug 28, 2017 at 21:44

Mario Lopes

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1 Comment

Mikkel Over a year ago

eval is fundamentally insecure :(

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