Concept: Discern between Array, Pseudo-Array, and Object in JavaScript

Question

Often while using JavaScript you run into silly problems. One such problems is discerning between Object types.

Is there a way to create a function with this functionality? See Below:

_discern = function () { [ function code ] };

_discern({}); // Logs: Object

_discern([]); // Logs: Array

_discern(document); // Logs: Pseudo-Object

_discern(document.querySelectorAll("*")); // Logs: Pseudo-Array

I've already tried creating a function that checks for array-likeness, but that didn't work as good as I had hoped:

isArrLike = function (_) {

    _[0] = 0; return [].slice.call(_).length >= Object.values(_).length;
};

And I've tried using that behavior into another function. All failures. Is there a way?

How do you define "pseudo-object"? Do you mean exotic object? There was a concept of host object, but it's been removed. A good start would be to provide an explicit definition of what tests or features define each type of object. — RobG
– RobG, Commented May 29, 2017 at 4:08

Logain · Accepted Answer · 2017-05-29 04:01:21Z

1

Looks like you want to know if something behaves like an array, instead of looking for the actual type/constructor.

If that's the case, it should be enough to check if something is iterable:

function isIterable(obj) {
  // checks for null and undefined
  if (obj == null) {
    return false;
  }
  return typeof obj[Symbol.iterator] === 'function';
}

answered May 29, 2017 at 4:01

Logain

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4 Comments

imaxwill Over a year ago

You're amazing.

RobG Over a year ago

What is your definition of "iterable"? Array methods work on any object with a length property, is that enough? Is a NodeList an "iterable"? It certainly is iterable. ;-)

Experimenter Over a year ago

@RobG Array methods DO NOT work on any object with a length property. E.g. arguments object of function. It has a length property and properties indexed from zero, but it doesn't have Array's built-in methods like forEach() and map()

RobG Over a year ago

@Experimenter— Array.prototype.call({length:1}, x => console.log(x)) works for me. And Array.prototype.forEach.call({length:1, 0:'blah'}, x => console.log(x)).

Taufik Nurrohman · Accepted Answer · 2017-05-29 04:11:39Z

0

All of these are objects:

is array → return x instanceof Array or Array.isArray(x)
is function → return typeof x === 'function'
is HTML element → return x instanceof HTMLElement
is plain object → return typeof x === 'object' && /* ... is not array, not function, not HTML element */

answered May 29, 2017 at 4:11

Taufik Nurrohman

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1 Comment

RobG Over a year ago

x instanceof Array will fail across frames, x instanceof HTMLElement makes assumptions about the host environment that aren't necessarily supported, as does typeof x === 'object' not being an HTML element or some other host object (try them in IE).

imaxwill · Accepted Answer · 2017-05-29 05:00:03Z

0

Thanks to Logain's answer, I can solve the problem. Here's my approach:

kind = function (a) {

    let u = toString.call(a.valueOf()).slice(8, -1);

    if (a == null || u == "String" || u == "Number" || u == "Boolean") {

        return u;
    }

    else if (typeof a[Symbol.iterator] == "function") {

        return u != "Array" ? "Array-Like" : u;
    }

    else {

        return u;
    }
};

answered May 29, 2017 at 5:00

imaxwill

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