Javascript - eval in context/scope without using this keyword without with statement

Question

This post provides a solution for how to use eval within a particular context/scope without having to preface all variables/functions in the eval'd code with this. javascript eval in context without using this keyword

To clarify - let's say I have the following object as the context for an eval statement {dog: "labrador"}. I'd like console.log(eval(dog)) to output "labrador" without having to type eval(this.dog)

However, the solution in the linked post:

function evalInContext(scr, context)
{
    // execute script in private context
    return (new Function( "with(this) { return " + scr + "}")).call(context);
}

uses the with statement. Considering usage of the with statement is discouraged, is there an alternative solution?

Why are you using this function at all? It looks like a long-winded, dangerous, and inefficient way to write: `context[scr]` — Quentin, Aug 05 '18 at 21:34
Apologies, that function was taken from the linked SO post as an example of how to refer to `dog` without doing `this.dog` when using eval in a particular context. I'm not using that function. — Foobar, Aug 05 '18 at 21:38
I still don't understand the problem you are trying to solve with eval — Quentin, Aug 05 '18 at 21:41
Please elaborate. `context[scr]` does respond to your "if I have an object... then I'd be able to ... without `this.dog`". So what is the problem? — Jeto, Aug 05 '18 at 21:41
@JonasW. I don't want to use `with` due to it being disabled in strict mode. — Foobar, Aug 05 '18 at 21:43
@Roymunson - That doesn't really clarify things. Why are you evaling code in the first place? `eval` is even more discouraged that `with`. — Quentin, Aug 05 '18 at 21:44
@roymunson on clientside: just set the variables in window maybe? on serverside: don't. — Jonas Wilms, Aug 05 '18 at 21:46
If you're sticking to `eval` (or `new Function()`), then sticking to `with` is not the issue. — trincot, Aug 05 '18 at 22:03
The term "context" has a particular meaning in JS, and it is not what you are using it for. — trincot, Aug 05 '18 at 22:14

score 1 · Accepted Answer · answered Aug 05 '18 at 22:37

1

is there an alternative solution?

Sure but thats way worse than with :

 return new Function(...Object.keys(context), scr)(...Object.values(context));

answered Aug 05 '18 at 22:37

Jonas Wilms

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Interesting, how does this work? I understand the `...` is spread notation - but I'm not sure how this expression executes code considering there is no call to `eval`. Also why would this be worse than `with`? – Foobar Aug 06 '18 at 00:11
1

@roymunson it creates a function with a few parameters and then calls that function passing in the parameters. It is worse than eval as no one will understand it – Jonas Wilms Aug 06 '18 at 09:38

Javascript - eval in context/scope without using this keyword without with statement

1 Answers1