How to execute a JavaScript function when I have its name as a string

I have the name of a function in JavaScript as a string. How do I convert that into a function pointer so I can call it later?

Depending on the circumstances, I may need to pass various arguments into the method too.

Some of the functions may take the form of namespace.namespace.function(args[...]).

Solution 1:

Don't use eval unless you absolutely, positively have no other choice.

As has been mentioned, using something like this would be the best way to do it:

window["functionName"](arguments);

That, however, will not work with a namespace'd function:

window["My.Namespace.functionName"](arguments); // fail

This is how you would do that:

window["My"]["Namespace"]["functionName"](arguments); // succeeds

In order to make that easier and provide some flexibility, here is a convenience function:

function executeFunctionByName(functionName, context /*, args */) {
  var args = Array.prototype.slice.call(arguments, 2);
  var namespaces = functionName.split(".");
  var func = namespaces.pop();
  for(var i = 0; i < namespaces.length; i++) {
    context = context[namespaces[i]];
  }
  return context[func].apply(context, args);
}

You would call it like so:

executeFunctionByName("My.Namespace.functionName", window, arguments);

Note, you can pass in whatever context you want, so this would do the same as above:

executeFunctionByName("Namespace.functionName", My, arguments);

Solution 2:

Just thought I'd post a slightly altered version of Jason Bunting's very helpful function.

First, I have simplified the first statement by supplying a second parameter to slice(). The original version was working fine in all browsers except IE.

Second, I have replaced this with context in the return statement; otherwise, this was always pointing to window when the target function was being executed.

function executeFunctionByName(functionName, context /*, args */) {
    var args = Array.prototype.slice.call(arguments, 2);
    var namespaces = functionName.split(".");
    var func = namespaces.pop();
    for (var i = 0; i < namespaces.length; i++) {
        context = context[namespaces[i]];
    }
    return context[func].apply(context, args);
}

Solution 3:

The answer to this other question shows you how to do that: Javascript equivalent of Python's locals()?

Basically, you can say

window["foo"](arg1, arg2);

or as many others have suggested, you can just use eval:

eval(fname)(arg1, arg2);

although this is extremely unsafe unless you're absolutely sure about what you're eval-ing.

Solution 4:

I think an elegant way of doing this is by defining your functions in a hash object. Then you can have a reference to those functions from the hash using the string. e.g.

var customObject = {
  customFunction: function(param){...}
};

Then you can call:

customObject['customFunction'](param);

Where customFunction will be a string matching a function defined in your object.

UPDATE

It seems that this answer was helpful for many fellow coders out there so here goes an updated version.

With ES6 you can additionally use Computed Property Names which will allow you to avoid magic strings.

const FunctionNames = Object.freeze({ 
  FirstFunction: "firstFunction", 
  SecondFunction: "secondFunction" 
});

...

var customObject = {
  [FunctionNames.FirstFunction]: function(param){...},
  [FunctionNames.SecondFunction]: function(param){...}
};

...

customObject[FunctionNames.FirstFunction](param);

Solution 5:

Could you not just do this:

var codeToExecute = "My.Namespace.functionName()";
var tmpFunc = new Function(codeToExecute);
tmpFunc();

You can also execute any other JavaScript using this method.