JavaScript introspection that is complete

Question

Beginner JavaScript question. I'm sort of spoiled on the dir built-in function from Python. I want to discover the properties/methods of any object in the node.js REPL. I have already seen this question; the accepted answer fails (in the node REPL) in the simple case of an empty array []. For example:

for(var prop in []){console.log(prop);}  # returns undefined, prints nothing
[].length  # returns 0

Since the for loop does not discover the array's length method, I do not consider that to be proper introspection. So, could someone fill in the blank here:

function magic(some_object) {
  # magic goes here
}

console.log(magic([]))  # should print a list that includes 'length'

Or is this simply not possible, or only possible for "user types"?

You can use tab completion in the REPL FYI and it works fairly well to see most things, but a complete answer to your question is fairly complicated. Mostly what you want can be achieved but it requires a fairly large bag of tricks. `util.inspect` and `console.log` in node are also handy, and I wrote my own `util.inspect` implementation at one point which I will post once I find it if an answer hasn't been accepted by then. — Peter Lyons, Jul 18 '13 at 20:33

score 11 · Accepted Answer · edited May 23 '17 at 10:31

11

How far back do you need to go in browser compatibility? All of the modern browsers should support Object.getOwnPropertyNames(). Using your example, Object.getOwnPropertyNames([]) will return ["length"].

More info here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/getOwnPropertyNames

Edit: Other examples:

Object.getOwnPropertyNames([1, 2, 3]); returns ["0", "1", "2", "length"]
Object.getOwnPropertyNames(String); returns ["prototype", "quote", "substring", "toLowerCase", "toUpperCase", "charAt", "charCodeAt", "contains", "indexOf", "lastIndexOf", "startsWith", "endsWith", "trim", "trimLeft", "trimRight", "toLocaleLowerCase", "toLocaleUpperCase", "localeCompare", "match", "search", "replace", "split", "substr", "concat", "slice", "fromCharCode", "length", "name", "arguments", "caller"]

Edit #2: Okay, so seeing that you are looking for complete list of properties and methods, including inherited ones, I've borrowed from two other SO questions (linked below) and come up with a solution that appears to get you even closer:

var findProperties = function(obj) {
    var aPropertiesAndMethods = [];

    do {
        aPropertiesAndMethods = aPropertiesAndMethods.concat(Object.getOwnPropertyNames(obj));
    } while (obj = Object.getPrototypeOf(obj));

    for ( var a = 0; a < aPropertiesAndMethods.length; ++a) {
        for ( var b = a + 1; b < aPropertiesAndMethods.length; ++b) {
            if (aPropertiesAndMethods[a] === aPropertiesAndMethods[b]) {
                aPropertiesAndMethods.splice(a--, 1);
            }
        }
    }

    return aPropertiesAndMethods;
}

So if you use call findProperties([]), it returns ["length", "join", "reverse", "sort", "push", "pop", "shift", "unshift", "splice", "concat", "slice", "lastIndexOf", "indexOf", "forEach", "map", "reduce", "reduceRight", "filter", "some", "every", "iterator", "constructor", "toSource", "toString", "toLocaleString", "valueOf", "watch", "unwatch", "hasOwnProperty", "isPrototypeOf", "propertyIsEnumerable", "__defineGetter__", "__defineSetter__", "__lookupGetter__", "__lookupSetter__"]

Linked Questions

javascript inheritance, reflection and prototype chain walking?

How to merge two arrays in Javascript and de-duplicate items

edited May 23 '17 at 10:31

Community

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answered Jul 18 '13 at 20:23

talemyn

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Well, I'm in `node` not a browser, but this is in the right direction. Still not giving me everything though. `Object.getOwnPropertyNames([])` returns `[ 'length' ]`, though an empty array also has a `.toString()` method. And `Object.getOwnPropertyNames(Array)` provides more methods (including `prototype`) but still not `toString`. You get my upvote but I'd still like to programmatically get *all* the properties/methods that are valid if I can. – wberry Jul 19 '13 at 02:46
1

Okay, so you are not only looking for the properties and methods of the actual object, but the ones that it inherits as well? Because, technically, `toString` is a method of `Object.prototype` and that is simply inherited by `Array`. Not sure if there is any way to grab the inherited methods and properties as well . . . – talemyn Jul 19 '13 at 15:28
1

Aha I think I'm learning here. So I could write a function that follows the inheritance tree up to `Object` and builds a set of everything it finds? I don't see an obvious way to do that. `Object.getPrototypeOf([])` just returns `[]` not `Array`. – wberry Jul 19 '13 at 16:00
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Funny you should ask . . . while you were typing that, I was writing that very function. :D Special thanks to some of the answers in the two threads that I linked for pointing me in the right direction. – talemyn Jul 19 '13 at 16:06
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Oh, and I added it to my answer, if that was not clear. LOL – talemyn Jul 19 '13 at 16:14
1

Your `findProperties` function appears to be exactly what I'm looking for. It shows me that `[]` has `push`, `toString`, etc. and that functions have `call` and `apply` properties. It still doesn't work for some things (`null`, `undefined`, `2`, `''`) but I assume that is because those things are not really considered to be objects and don't have properties anyway. For example while it doesn't work for `''` it does work for `new String('')` which is considered different. – wberry Jul 19 '13 at 16:59
3

You can get a return for `2` if you make it a defined `Number` object (i.e., `console.log(findProperties(new Number(2)));`). Interestingly, it gives you a different result than `console.log(findProperties(Number));`, showing the difference between an object (e.g., `Number`) and an instance of an object (e.g. `2`). – talemyn Jul 19 '13 at 17:07
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I upvoted this years ago. I wish I could upvote it again because I use it all the time! – Mark McClure Apr 14 '18 at 13:34
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@MarkMcClure - Glad it helped! Honestly, it's probably the most interesting conversation that I've ever had on SO. :D – talemyn Apr 16 '18 at 17:42

JavaScript introspection that is complete

1 Answers1