JS Check if input string is valid code and run input

Question

So I really couldn't find much on this. The idea is that I have a user input (text) and I want to check if its valid code in JS and then run it if it is.

I know that I can do if (typeof userInput === "function") { if the userInput was actually a function, but the user input is just a string input at this point, so I'm quite stuck, and it also contains any arguments or other code.

As an example, the input may be "alert('test')" and that would be valid, and I would like to run that, however something like "madeUpFunction(lol)" would be invalid.

I'm happy if I can just get functions working and not any JS code.

What you're trying to do amounts to parsing JavaScript syntax. You can let the JavaScript runtime itself do it vial `eval()`, but that's obviously not without risk and complications. — Pointy, Nov 17 '16 at 17:21
@MattCowley Function !== function call. It's not clear at all what you have and what you want. — Oriol, Nov 17 '16 at 17:26
What are you trying to achieve with this? Do you consider things like `1,alert('test')`, `(1,alert)('test')`, `1;alert('test')`, `{alert('test')}` to be function calls? Are you trying to validate a [CallExpression](http://www.ecma-international.org/ecma-262/7.0/#prod-CallExpression)? — Oriol, Nov 17 '16 at 17:44
@Oriol I'm not sure, basically if it's valid JS that can be run, I want to run it. — Matt Cowley, Nov 17 '16 at 17:45

4castle · Accepted Answer · 2016-11-17T18:37:49.993

2

You can extract the string up until the first ( in order to get the function name, and then test that it exists using typeof eval(funcName). Then use the Function constructor to make sure that the syntax is valid for the rest of the arguments without actually executing the code.

You can then return the function which executes what the user entered, but note that there may be security issues if you decide to execute the function.

function parseFunctionCall(str) {
  var funcName = str.slice(0, str.indexOf('('));
  if (typeof eval(funcName) === 'function') {
    return new Function(str);
  } else {
    throw new Error('Invalid function name');  
  }
}

console.log(parseFunctionCall("console.log('hi')"));

edited Nov 17 '16 at 18:37

answered Nov 17 '16 at 18:01

4castle

32,613
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This is great, thank you! However if I input `console.log('test')` it doesn't detect it? – Matt Cowley Nov 17 '16 at 18:33
That's because `console.log` is selecting a function that isn't directly on `window`. I've updated to use `eval`, but [there are more efficient ways to do it](http://stackoverflow.com/q/6491463/5743988). – 4castle Nov 17 '16 at 18:40

score 0 · Answer 2 · answered Nov 17 '16 at 17:57

You can use eval("typeof "+ input) to achieve what you want. The argument of the eval() function is a string. If the string represents an expression, eval() evaluates the expression.

var myFunc = function(){};
var input = prompt('Enter input string');

if(eval("typeof "+input) === "function"){
 document.getElementById('d1').innerHTML = "Yes it's a funtion";
}

else{
  document.getElementById('d1').innerHTML = "Not a funtion";
}

<div id="d1">

</div>

score -1 · Answer 3 · answered Nov 17 '16 at 17:22

-1

Assuming this is in a browser and that that function you want to check is in the global scope:

if (typeof window[userInput] === "function" ) {
    ...
}

answered Nov 17 '16 at 17:22

Chris Charles

4,406
17
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JS Check if input string is valid code and run input

3 Answers3