Syntax error on eval() with json as string

Question

I'm getting an error [Exception: SyntaxError: Unexpected token :] when I try to evaluate the following expression:

eval("{"T1": [1,2,3,4,5,6,7,8,9,10,11,12], "T2": [12,11,10,9,8,7,5,4,3,2,1]}")

However, the same expression without the " works:

eval({"T1": [1,2,3,4,5,6,7,8,9,10,11,12], "T2": [12,11,10,9,8,7,5,4,3,2,1]})

If my JSON is in string format, like in the first example, how can I convert it into a javascript object?

If I try using:

JSON.parse("{"T1": [1,2,3,4,5,6,7,8,9,10,11,12], "T2": [12,11,10,9,8,7,5,4,3,2,1]}")

I get [Exception: SyntaxError: Unexpected identifier]. How can I escape the "?

That isn't JSON. If it was, T1 and T2 would be surrounded by " not '. Fix that and use a real JSON parser, not eval. — Quentin
– Quentin, Commented Mar 29, 2012 at 15:53

Community · Accepted Answer · 2017-05-23 12:04:29Z

5

Avoid using eval (see why), use JSON.parse when available. To support older browsers, I suggest using a third-party library like Crockford's.

On your second example, it works because there is nothing to be parsed, you already have an object.

EDIT: you added one more question, here is the answer: you escape " with \. For example, this is a valid string containing just a quote: "\"".

edited May 23, 2017 at 12:04

CommunityBot

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answered Mar 29, 2012 at 15:52

bfavaretto

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Bergi · Accepted Answer · 2012-03-29 15:54:23Z

3

The curly braces are interpreted to be a code block, not a object delimiter. Therefore you'll get an exception for the colon.

You can work around that by surrounding your object with (), but better use JSON.parse. Don't forget: eval is evil :-)

answered Mar 29, 2012 at 15:54

Bergi

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Mike Samuel · Accepted Answer · 2012-03-29 15:54:05Z

1

You need parentheses, but really, use JSON.parse as bfavaretto suggested.

To understand why your current code is failing, consider that

eval("{}")

runs the program

{}

which is just a block containing no statements while

eval("({})")

runs the program containing a single statement which evaluates the expression {}, an empty object.

answered Mar 29, 2012 at 15:54

Mike Samuel

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KDV · Accepted Answer · 2012-03-29 15:56:27Z

0

What's the point of just evaluating an json object without actually assigning it to any variable?

It seems kind of pointless to me.

eval isn't a dedicated json parser. It's JS parser and it's assuming that the {} is a block of code.

This however works

eval("var abc = {'T1': [1,2,3,4,5,6,7,8,9,10,11,12], 'T2': [12,11,10,9,8,7,5,4,3,2,1]};");

As everyone stated, use JSON.parse if you really need to parse json. Eval is dangerous.

answered Mar 29, 2012 at 15:56

KDV

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