Why 'eval' execution gives error?

Question

I have following code in HTML part:

<script>
    var trainings = {'M':new PlayerTraining(18369,22,355),'D':new PlayerTraining(9412,6,118)};
</script>

This code works well, and 'PlayerTraining' is class defined in one of js-files referenced above.

But when I execute:

<script>
    alert(eval("{'M':new PlayerTraining(18369,22,355),'D':new PlayerTraining(9412,6,118)}"));
</script>

I receive the following error:

Uncaught SyntaxError: Unexpected token :

What am I doing wrong? In the end I need to assign that hash (received from WebService) to variable. How to do that?

Please advise.

`eval("{'M':new PlayerTraining(18369,22,355),'D':new PlayerTraining(9412,6,118)}")` is (more or less) the same as `` -> SyntaxError — Andreas, Jul 20 '15 at 07:14
The `{` is seen as the start of a block. The following up to the closing `}` is expected to be a sequence of statements. 'M' is a string literal and `:` is a syntax error. — RobG, Jul 20 '15 at 07:24

score 1 · Answer 1 · edited May 23 '17 at 12:21

1

You can't eval objects like that.

Similar to your example, this will throw a syntax error:

{ a: new Date(), b: new Date() }

This is because you're missing a variable assignment:

var x = { a: new Date(), b: new Date() }

You shouldn't be using eval in the first place, though. It's unsafe, and inefficient.

edited May 23 '17 at 12:21

Community

answered Jul 20 '15 at 07:20

Cerbrus

I received JSON data from web service, so I consider that as efficient. Is that safe? Dunno, hope yes. See no concerns so far. – Budda Jul 21 '15 at 01:10
Oh, save would be to parse string... got it. Agree, thanks – Budda Jul 21 '15 at 01:12

Oleg · Accepted Answer · 2015-07-20T15:35:39.497

1

To make it work you should wrap your expression in the parentheses:

alert(eval("({'M':new PlayerTraining(18369,22,355),'D':new PlayerTraining(9412,6,118)})"));

and to assign the result to a variable do it like this:

var someVar = eval("(" + webServiceResponse + ")");

edited Jul 20 '15 at 15:35

answered Jul 20 '15 at 07:20

Oleg

Luca Rainone · Answer 3 · 2015-07-20T07:27:20.130

0

Inside eval assign your object to a var:

eval("var myVar = {'M':new PlayerTraining(18369,22,355),'D':new PlayerTraining(9412,6,118)};");
alert(myVar);

~~But anyway, If you have the object as string, you can parse as json.~~

edited Jul 20 '15 at 07:27

answered Jul 20 '15 at 07:19

Luca Rainone

`JSON.parse` only accepts valid JSON. Those functions calls aren't valid JSON. – Cerbrus Jul 20 '15 at 07:23
@Cerbrus correct. Sorry – Luca Rainone Jul 20 '15 at 07:26

3 Answers3