Call javascript function encoded in a string

Question

If I put a function into a string like this:

var functionString = function (message) {
    console.log(message);
}.toString();

Is there any way to convert the string back to a function and call it? I tried

eval(functionString)

which returns "Uncaught SyntaxError: Unexpected token", and

functionString.call(this, "HI!");

which returns 'undefined is not a function'.

Is that even possible in javascript?

Thanks in advance for any reply!

EDIT: The point of this question is that the function has been converted into a string using toString(). So

console.log(functionString);

returns this string: "function (message) {console.log(message);}"

Can I transform the string back into a function and call it? That's the problem I am trying to solve. Thanks!

You could just do "return message.toString()" inside the function — ryanc1256
– ryanc1256, Commented Dec 5, 2014 at 21:12
Is it possible, yes it can be, but what are you trying to do exactly? — epascarello
– epascarello, Commented Dec 5, 2014 at 21:14
epascarello, I would like to convert the string back to a function. Brian, I don't want to call an existing function. I would like to convert the entire string into a function. — user1840267
– user1840267, Commented Dec 5, 2014 at 21:20

Aaron Hammond · Accepted Answer · 2014-12-05 21:20:59Z

3

You're nearly there, but you're missing something.

When we call toString() on your function, we get

"function (message) {
    console.log(message);
}"

which we can then eval. However, we're just creating an anonymous function object here; we won't be able to call it!

If we instead to something like:

var functionString = "var restoredFunc = " + function (message) {
    console.log(message);
}.toString();

We can then do the following

eval(functionString);
// prints "hello!" to console
restoredFunc("hello!");

answered Dec 5, 2014 at 21:20

Aaron Hammond

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Comments

Alex Pakka · Accepted Answer · 2014-12-05 21:37:10Z

2

Your functionString contains exactly the string

"function (message) { console.log(message); }"

Evaluating it as-is does present JavaScript engine with incorrect syntax (there is no name for this function). JavaScript expects construct like function <name>(<params>) { }. Alternatively, you can use anonymous function (i.e. no name present), but only as a parameter or in a context of evaluating expression. The minimal typical evaluating expression would be (function() {})() If you want to get fancy, !function() {} is also ok - the exclamation mark in front turns it into boolean expression that requires function evaluation before negating the output.

So, in your example this will work:

eval("("+functionString+")('abc')");

because then you do anonymous function call - something JavaScript can live with.

Alternatively, you can also use just brackets, then you need to assign the result to something you can use later:

var foo = eval("("+functionString+")");
foo('ddd');

Here is a little proof / playground to learn about it: http://jsfiddle.net/Exceeder/ydann6b3/

edited Dec 5, 2014 at 21:37

answered Dec 5, 2014 at 21:20

Alex Pakka

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3 Comments

user1840267 Over a year ago

Works, thanks! Could you tell me what I should google for to learn more about this, for example the +-wrapping of functionString?

Alex Pakka Over a year ago

@user1840267 for eval(), it always makes sense to print the actual string you evaluate, then copy-paste it in the editor of your choice in empty .js file and see if it is syntactically correct.

user1840267 Over a year ago

Ah, I see, he is simply wrapping the function into parenthesis! Got it, thanks!

tinker · Accepted Answer · 2014-12-05 21:23:59Z

1

yes its possible in JavaScript but you can't eval anonymous function without assignment

So you go through it like so

var functionString = function (message) {
    console.log(message);
}.toString();

eval("myfunction =" + functionString)

myfunction("Hello World!")

answered Dec 5, 2014 at 21:23

tinker

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Comments

Kevin Hakanson · Accepted Answer · 2014-12-05 21:24:03Z

1

Your functionString is a string that looks like

"function (message) {
    console.log(message);
}"

You could covert that string to an Immediately-Invoked Function Expression (IIFE) using string concatenation, similar to below.

(function (message) {
        console.log(message);
})("HI!");

and then eval that. Here is the result from Chrome's JavaScript console:

eval('(' + functionString + ')("HI!")')
HI!

answered Dec 5, 2014 at 21:24

Kevin Hakanson

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