Inspect the names/values of arguments in the definition/execution of a JavaScript function

StackOverflow https://stackoverflow.com/questions/914968

Question

I'm looking to do the equivalent of Python's inspect.getargspec() in Javascript.

I do know that you can get the arguments list from a Javascript function, but what I'm really looking for is the names of the arguments from the originally defined function.

If this is in fact impossible, I'm going to have to 'brute-force' it by getting the string of the function, i.e. myFunc.toString() and then parsing out the ... inside of function myFunc(...). Does anyone know how to do this parsing in a general way?

Thanks for any tips.

Was it helpful?

Solution

While I can't see any good reason for this, 

var reg = /\(([\s\S]*?)\)/;
var params = reg.exec(func);
if (params) 
     var param_names = params[1].split(',');

assuming func is the name of your function

OTHER TIPS

here a small nodejs module for that.

https://github.com/kilianc/node-introspect

The following function will return an array of parameter names of any function passed in.

function getParamNames(func) {
    var funStr = func.toString();
    return funStr.slice(funStr.indexOf('(')+1, funStr.indexOf(')')).match(/([^\s,]+)/g);
}

Example usage:

getParamNames(getParamNames) // returns ['func']
getParamNames(function (a,b,c,d){}) // returns ['a','b','c','d']

Here's a version that uses higher order functions. The only thing I don't like about it is that you have to specify parameters as members of an array. But at least they only have to be specified once.

<!DOCTYPE html>

<html>
<head>
<meta charset="UTF-8">
<title></title>
<script type="text/javascript">
var withNamedParams = function(params, lambda) {
    return function() {
        var named = {};
        var max   = arguments.length;

        for (var i=0; i<max; i++) {
            named[params[i]] = arguments[i];
        }

        return lambda(named);
    };
};

var foo = withNamedParams(["a", "b", "c"], function(params) {
    for (var param in params) {
        alert(param + ": " + params[param]);
    }
});

foo(1, 2, 3);
</script>
</head>
<body>

</body>
</html>

Sightly improved version of Jonathan Fingland's answer:

// Utility function to extract arg name-value pairs
function getArgs(args) {
    var argsObj = {};

    var argList = /\(([^)]*)/.exec(args.callee)[1];
    var argCnt = 0;
    var tokens;

    while (tokens = /\s*([^,]+)/g.exec(argList)) {
        argsObj[tokens[1]] = args[argCnt++];
    }

    return argsObj;
}

// Test subject
function add(number1, number2) {
    var args = getArgs(arguments);
    alert(args.toSource()); // ({number1:3,number2:4})
}

// Invoke test subject
add(3, 4);

Note: This only works on browsers that support arguments.callee.

(Also posted at How to get function parameter names/values dynamically from javascript)

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