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Javascript Get flat array from object contain multiple arrays

I'm trying to merge multiple arrays from one object using JavaScript(React-Native), the example of the object is :
{"10419": ["37046", "37047"], "9138": ["32809"]}
The result should be like so:
["37046","37047","32809"]
I need to ignore the object name and end up with only one flat array
I used flat() function but it seems not working as I need.
my try looks like :
var obj = this.state.obj // will contain the object
console.log(obj.flat()) // I know that work only with arrays but I tried it out
Thanks

Using Object.values, you can get the values inside the object. That will be 2d array from your input.
Using Array.prototype.flat, you can make it as 1d array as follows.
const input = {"10419": ["37046", "37047"], "9138": ["32809"]}
const result = Object.values(input).flat();
console.log(result);

Object.values(this.state.obj).flat()

Trouble with basic Javascript

I haven't touched Javascript in a while and now I'm having trouble with basic arrays.
params=new Array();
params['return']='json';
alert(params.length);
This always returns 0 when I'm expecting 1. What's wrong with this?

Arrays use numerical indexes. When you use a string "index", it just adds a new property to the array object, but the new value isn't in the array.
Thus, the array is still empty, but you can access your value as you could with any other object.
Your code is equivalent to
var params = {}; //new object (not array)
params['return']='json'; //new property added to object

A few things:
You forgot var:
var params = new Array();
But an array takes numeric indices, so params['return'] is not really a member of the array but of the object that represents the array, so it doesn't affect the length.
You could use an object but objects have no length:
var params = {};
params['return'] = 'json';
To get the length though you can count the keys in that object and get the length of the resulting array:
var len = Object.keys(params).length;

Javascript arrays don't hold key value pairs like objects do, so the length isn't incremented when you assign a value. They are however objects themselves, so you can assign values to its properties (in this case the return property).
You probably want params to be a plain object: params = {}
You need to count the properties, like this for example:
function dictionarySize(dict){
var size = 0;
for (key in dict){
// In practice you'd probably want to filter using hasOwnProperty
size++;
}
return size
}
alert(dictionarySize(params));
Or using a library like underscore.js:
_.size(params);
Object.keys is also an option, although it won't work in IE8 and older.
If you don't need an key value pairs use params.push:
params=new Array();
params.push('json')
alert(params.length); // 1

You can create an array, push stuff on it, and assign properties to values of it like so:
var params=[];
params.push('firstly');
params[0]="jonson";
params['return']="fredy"
params.newone="json";
params.push('Carl');
NOW, if you do:
console.log(params.length,params);
the result of that is:
2 ["jonson", "Carl", return: "fredy", newone: "json"]
SO, you see "firstly" was replaced by "jonson" in the [0] - so the "pushed" value is addresed by the numerical [0]

Javascript multidimensional arrays with alphanumeric keys

This seems to be a common source of confusion from what I've seen, and apparently I'm no exception. I've read a few tutorials on this, and I still can't quite get my head around it. From what I can gather, Arrays are objects in Javascript, just like Strings and other variable types. But I still don't get how that helps me declare a multidimensional array with alphanumeric keys.
In PHP I can simply write:
$calendar = array();
foreach ($schedule->currentExhibitions as $key) {
$calendar[$key["ExhibitionID"]]["startDate"] = date("Y,n,j", strtotime($exhibition["StartDate"]));
$calendar[$key["ExhibitionID"]]["endDate"] = date("Y,n,j", strtotime($exhibition["StartDate"]));
}
But in Javascript trying something similar will create errors. Should I create an Array and fill it will Objects? If so, how would I go about doing so? Or should I just use an Object entirely and skip having any sort of Array? (If so, how do I create a multidimensional Object?)
Sorry for the newbish quesion!

If your keys are strictly numerical and ordered starting at zero, then an array makes sense and you can use square bracket notation just like you would in php, although you will need to initialize sub-arrays if you want to have multiple dimensions :
var myArray = [];
myArray[0] = [];
myArray[0][0] = "something interesting";
If your keys are not numerical, ordered and starting at zero, then you should use an object (all keys are strings), which still allows the square bracket notation :
var myObject = {};
myObject["1A"] = {};
myObject["1A"]["3B"] = "something interesting";

In Javascript, an array is an object, who's keys are numerical, sequential, indexes.
As soon as you want to use alpha-numerica (aka strings) keys, you use a regular object.
In JS to do what you want, you'd do the following (using more or less your php code).
var calendar = {};
Object.keys(schedule.currentExhibitions).forEach(function(key) {
var ex = schedule.currentExhibitions[key];
calendar[ex.exhibitionId] = calendar[ex.exhibitionId] || {}; //if the key doesn't exist, create it.
calendar[ex.exhibitionId].startDate = date(); //some js date function here
calendar[ex.exhibitionId].endDate = date(); //your js date function here
});

I look at Multidimension as nesting, and at multiple levels of nestings as complex objects. For example:
var parent = [];//top holder
var child1 = {};
child1.name = "Jim";
parent.push(child1);
In this simple example, you can access child1 like this:
parent[0]["name"] //Jim
So that is, in a way, multidemensional. Instead of using ["name"] as an indexer, or child1 as an object it could also be an array, like this:
var parent = [];//top holder
var child1 = [];
child1.push("Jim");
parent.push(child1);
In this example, you could get Jim with:
parent[0][0];//Jim
So for complex examples you may have multiple levels of these nestings (or dimensions).
parent[0]["Child"].grandChild[5]["cousin"].name //etc
Where that would just be a continuation of the previous examples down the line.

If you want to preserve order or you want to access by numeric index, use an array. The value of the array can be a single value or an object or array itself (so each value in the array can contain more than a simple value).
If you want to access by a unique alphanumeric key, then use an object and assign properties to it.
Arrays have numeric indexes. They do not have alphanumeric indexes.
Objects have string keys.
Because an array is also an object, it can have both types of keys, but using a string key is not an array access, it's accessing a property of the object.
When you ask for the .length of an array, you only get the length of the numeric indexes. It does not include other properties of the object.
An array of objects is a very practical data structure in javascript and is used quite often when either order or index by numeric index is important.
If order is not important or you don't need to access by numeric index and just want to access by an alpha numeric string, then you should just use an object and set a properties on it with keys that are your alphanumeric string.

My associative array doesnt store objects

It seems it's always empty :
var idStruttura=2;
var arrayMarkers=new Array();
arrayMarkers["sede_"+idStruttura] = "ciao";
alert(arrayMarkers.length);
Prints always 0. Why? And how can I fix it?

An array is not an associative array, however the array object is an object, and all objects are associative arrays. If you use a string as key when assigning items to the array, you are not using it as an array, you are using it as an object.
The length property of the array returns how many items you have stored in the array. If you also use the array object as an associative array, that won't affect how you use the array as an array.

What you've created is a regular array object and added a property to it called sede_.... JavaScript doesn't use associative arrays in the same way a language like PHP does. Arrays are objects that can have properties, but those properties are not among the numerically indexed array elements.
var idStruttura=2;
var arrayMarkers=new Array();
// Push an object onto the array having one property:
arrayMarkers.push({"sede_" + idStruttura : "ciao"});
// Or declare it as an object to begin with:
// This makes more sense....
var objMarkers = {};
objMarkers['sede_' + id] = 'ciao';

There is no length when you store objects, only when you use the array as intended.
Try this (DEMO)
var idStruttura=2;
var arrayMarkers={}; // creates a more appropriate object than []
arrayMarkers["sede_"+idStruttura] = "ciao";
arrayMarkers["sede_"+(++idStruttura)] = "espresso";
var len = 0;
for (var o in arrayMarkers) {
if (arrayMarkers.hasOwnProperty(o)) len++;
}
arrayMarkers.length=len
alert(arrayMarkers.length)

array's in javascript (unlike php) cannot have string key's, only numeric keys. If you want a string literal as a key, please use objects

What you need is a simple Object, not an Array. Arrays have numeric indexes.
var markers = {};
markers['sede_' + id] = 'ciao';
Also note that {} is the same as new Object() and [] is the same as new Array(). Always use the former ones. It's much clear to any JS developer.

push associative array error?

I have to push elements in an associative array from another array after some processing and I'm doing something this:
for(var i in this.aNames) {
var a = this.aNames[i];
// some processing on a
aList[i] = a;
aList.push(i);
}
But it's not giving me the proper array.
EDIT :
Here aNames is an associative array like this
'1232':'asdasdasd',
'6578':'dasdasdas'
...... and so on of about 100 elements.
I'm using for here as I want to do some changes in every element of the array.
Then I'm displaying the result array on the page but it's showing the key value together with the array data.
I.e. it should only display asdasdasd or asdasdasd but it's displaying keys too, like 1232 asdasdasd 6578 dasdasdas.

There are multiple things which may go wrong...
Primarily, make sure that this is pointing to the correct context and that this.aNames is actually returning a complex object (associative array).
Also, what's aList? Is it an array? If it is, push should append your array with the key of the current member (the member's name).
If you want to append the values of the members on your source object, you need to do something like this:
var obj = {name: 'dreas'},
arr = []; // arr is an array
arr.push(obj["name"]); // arr now contains a single element, 'dreas'
In your for..in construct, you are both adding elements to an alleged array (aList) with push but also creating new members on your array (with the subscript notation, aList[i] = "asd" since i in this case (for..in iteration) refers to the member's name).
So, what you need to do is decide if you want to add elements to an array or members to an object, not both.
If you just want to clone an array, use a for loop. If on the other hand you want to clone an object, it's not that trivial because members can also be complex objects containing their own members, and simply doing arr[i] = obj.member will only copy a pointer to arr[i] if member is a complext object, not a value type.
Just to make sure my terminology is understandable:
var anObject = {name: "dreas"},
anArray = [1,2,3];
anObject["name"] <= member (results in "dreas")
anArray[1] <= element (results in 2)