Why eval is not working in this snippet?

Question

I would like to know whats wrong with following JavaScript code -

<html>
<script type="text/javascript">

var p = eval('new { "Color":"Red"}');

alert(p.Color);
</script>
</html>

It is giving me JavaScript error as -

Message: Object doesn't support this action
Line: 4
Char: 1
Code: 0

Not answering your question but why not: var p = { "Color":"Red"}'? — Mikael Härsjö, Apr 10 '12 at 09:35
What are you trying to achieve? Why is `var p={"Color":"Red"}` not good enough? — Niet the Dark Absol, Apr 10 '12 at 09:41
@Kolink- that code will be embedded from server side coding and it is dynamic. — Parag Meshram, Apr 10 '12 at 10:10

danwellman · Accepted Answer · 2012-04-10T10:28:49.910

There are several problems with the code;

firstly you're telling the script to create something new, but you're not telling it what to create. If you aren't creating a custom object, you are creating an Object object, so you need to tell it that:

var p = eval('new Object()');

Now that you are creating a new Object object, you can configure the Color property:

var p = eval('new Object({"Color":"Red"})');

Here is a working fiddle

But, why are you using eval in the first place? eval is evil!

score 1 · Answer 2 · edited May 23 '17 at 11:51

Why eval is not working in this snippet?

Because you cannot use the new operator ^[MDN] with objects, only with functions.

{"Color":"Red"} is evaluated as object literal and not as function.

If you want to parse JSON, use JSON.parse ^[MDN]. See also JSON.parse vs. eval().

Or create the object literal directly, if it is static:

var p = {"Color":"Red"};

To make it work with eval (which you should avoid to use anyway), remove new and wrap the literal in parenthesis:

var p = eval('({"Color":"Red"})');

The parenthesis are necessary because otherwise JavaScript would evaluate {"Color":"Red"} as a block and throw an error.

Why eval is not working in this snippet?

2 Answers2