Eval doesn't work - JavaScript

Question

I've never used eval() before, so I assume that I just got the syntax horribly wrong. What's wrong with the following:

var JSONAsString = '{"item1":"one", "item2":"two", "item3":"three"}';
var JSONAsObject = eval(JSONString);
alert(JSONAsObject.item1);

Since it doesn't seem to be working - I load the page and nothing happens.

And yes, I know I shouldn't be using eval. I assume that the syntax for JSON.parse() is the same as that of eval... right? If it is, if (after fixing the code) I replace eval with JSON.parse, will it still do the same thing?

JSONString vs JSONAsString, JSONAsObject vs JSONObject - your variable names are incorrect. Besides that, after parsing/evaluating it it's a JavaScript object, not JSON (just to be nitpicking ;)), so JSONObject doesn't really fit. — ThiefMaster
– ThiefMaster, Commented Jun 17, 2012 at 10:09
Assuming you have the variable names correct, evaling the string results in a syntax error because the {...} are interpreted as block and not as object literal. — Felix Kling
– Felix Kling, Commented Jun 17, 2012 at 10:11

ThiefMaster · Accepted Answer · 2012-06-17 10:06:34Z

5

When using eval you need to wrap the JSON in ():

var JSONAsString = '{"item1":"one", "item2":"two", "item3":"three"}';
var JSONAsObject = eval('(' + JSONAsString + ')');
alert(JSONAsObject.item1);

However, you should use JSON.parse() right from the beginning, not just later. Otherwise possibly invalid JSON that is valid JavaScript might work but stop working when switching to JSON.parse.

Note that you should include json2.js when using JSON.* since some older browser do not have native JSON support.

answered Jun 17, 2012 at 10:06

ThiefMaster

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6 Comments

Bluefire Over a year ago

Aah, thanks! Does that go for all code that I put into a variable and then parse using eval(), or just JSON?

MaxArt Over a year ago

Use JSON.parse() and forget the rest.

ThiefMaster Over a year ago

What @MaxArt says. But in any case, if you eval() an expression you'll want () around it to ensure it's not evaluated as a statement.

Bluefire Over a year ago

So yeah, about JSON.parse - does it require similar syntax? So, again, if I went into some code which used eval to parse JSON and replaced it with JSON.parse, without changing anything else, would that still be fine?

ThiefMaster Over a year ago

Yes, you simply use JSON.parse(some_string_containgin_json)

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Emil Stenström · Accepted Answer · 2012-06-17 10:08:24Z

3

Don't use eval() to parse JSON. Use Douglas Crockfords json2, which gives you cross-browser support, performance and security: https://github.com/douglascrockford/JSON-js

answered Jun 17, 2012 at 10:08

Emil Stenström

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