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I have a little problem when I want to generate a json string containing javascript function.
I am using json_encode() function and it works well for encoding any object/array to be json string. But, I want to fill one of an element value by a javascript function.
json_encode() will always encode my function to string. How do I can remove the "(quote)? The quote" makes my function be a string. here's the example:

$var['status']     = "oke";
$var['fnFeedback'] = "function (a,b,c){ return a+c.FnSource; }";

echo json_encode($var);
//then the result
{[status:"oke"],[fnFeedback:"function (a,b,c){ return a+c.FnSource; }"]}
//value of fnFeedback of above json is a string

How can I make value of fnFeedback be a function?

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  • 2
    you can't (directly) - there's no specific JSON encoding for functions. Commented Oct 21, 2014 at 7:10
  • JSON cannot contains functions Commented Oct 21, 2014 at 7:10
  • so I have to remove the quotes manually? Commented Oct 21, 2014 at 7:11
  • 2
    no, since your JSON decoder will do that for you, leaving a string. You'll then have to recognise that the string contains a function body and then pass it either to new Function() or eval (bleuch) to turn it into an actual function. Commented Oct 21, 2014 at 7:12
  • 1
    Seriously: don't. Passing functions as JSON strings means using the function constructor or eval, both of which make it really easy to exploit your code Commented Oct 21, 2014 at 7:18

3 Answers 3

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First, your sample isn't legal JSON syntax. Suppose it's something like this instead:

var input = '{"status":"oke","fnFeedback":"function (a,b,c){return a+c.FnSource;}"}';

One way to deal with this is to parse normally and then use eval to turn the value of the fnFeedback property into a Function object. However, using eval is usually a last resort (not to mention extremely risky if you aren't in total control of the value being eval-ed) and here there's a better approach using a little trick when parsing JSON data.

The trick is that JSON.parse accepts a second argument, which should be a function that accepts a key/value pair and returns the result. This will be called for every element that is parsed. Here's one that will turn your serialized function string into an actual function:

var result = JSON.parse(input,
    function(key, value) {
        // if value looks like a function definition, make a function
        if (value 
                && typeof value === "string" 
                && value.substr(0,8) == "function")
        {
            var startBody = value.indexOf('{') + 1;
            var endBody = value.lastIndexOf('}');
            var startArgs = value.indexOf('(') + 1;
            var endArgs = value.indexOf(')');

            return new Function(value.substring(startArgs, endArgs),
                                value.substring(startBody, endBody));
        }
        // else just return the value as is
        return value;
    });

When this is run, you can then see the results:

> result['fnFeedback']
[Function]
> result['fnFeedback'].toString()
'function anonymous(a,b,c) {\n return a+c.FnSource; \n}'

A more detailed explanation of how this works can be found here (from which the above was adapted). Also see this JavaScript gist for another (perhaps cleaner) sample code using the same idea (which I have not tested).

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very nice answer - I didn't have the time to explain the difference between Function and eval earlier.
2

You can't. JSON means: JavaScript Object Notation. It is a serialisation to String values of all properties of an Object. Function properties are not serializable. If you deserialize the JSON-string to Object you'll have to convert the string containing the function to a real javascript function (e.g. using eval or new Function).

See also ..., chapter 5:

A JSON value can be an object, array, number, string, true, false, or null

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6 Comments

or even just json.org
nit-pick: "A JSON value can be an object, array, number, string, true, false, or null" <-- but a function is an object!
@EliasVanOotegem yes, but it's a special kind of object, that has a constructor, and a prototype chain.
@Alnitak: I know, just failing to be funny ;-P
oke, Thanks for the reference! I'll try to find the solution
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JSON cannot directly represent function bodies.

Given your JSON converted back into a JS object (below called myObj) one possibility is to convert the string directly into a function by wrapping it in parentheses (thus converting the function declaration into a function expression) and then passing it to eval:

var fnFeedback = eval('(' + myObj.fnFeedback + ')';

Please note that there are possible security issues with using eval - use with extreme caution and do not trust any function body supplied indirectly by third parties. It's safer to use Function instead, see Ted Hopp's later answer.

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3 Comments

and re-assign the fnFeedback to the json, done! :D
@OkiErieRinaldi please accept Ted Hopp's answer - the version using Function is far preferable to using eval but I didn't have time to write that version earlier.
Yes, his is more complex, and it is clearer to be understood, but i didn't use it. I just need a little trick to go. But for a common problem, I think Ted Hopp's answer is preferable (as your suggest).

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