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Javascript: Subarrays contain reference to other arrays even though parent is its own array? [duplicate]

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OK.
Let me try to say this in some sort of comprehensible manner. I'm an amateur programmer writing my own take on a neural network in javascript. (Without having seen the code for a neural network before)
I was having problems with an array changing when I was trying to change a copy of the array. (Not the original)
I soon realized after rereading what I'd written that when you assign an identifier to an array it doesn't make the identifier a new object with a copy of the array. Instead, it makes a reference to the original array object, for example:
var arr = [1,2,3];
var arr2 = arr;
arr2[0] = 9;
alert(arr);
//Alerts "9,2,3"
With this mind, I googled and found a quick solution:
var arr = [1,2,3];
var arr2 = arr.slice();
arr2[0] = 9;
alert(arr);
//Alerts "1,2,3"
So I changed this in my actual project expecting to see my work completed, but no! I was getting the exact results as before where my array was changing even though it was not supposed to.
After much effort at debugging, I finally worked out that the problem here is that I have a large array of subarrays, which in turn have subarrays.
In code this looks like:
var arr = [
[[1],[2]],
[[4],[5]],
[[7],[8]]
];
As you can see, there is one big array that contains 3 smaller arrays, each of which contains two even smaller arrays, each of which contains a number.
In my project, it's more complicated than this and has a couple more layers but this is a decent representation.
So what did I expect to happen?
var arr = [
[[1],[2]],
[[4],[5]],
[[7],[8]]
];
var other = arr.slice();
other[0][0][0] = "Uh Oh";
alert(arr);
//Outputs "1,2,3,4,5,6,7,8"
What actually happened?
alert(arr);
//Outputs "Uh Oh,2,3,4,5,6,7,8"
Why does this happen?
How can I fix it?
Thanks in advance.

Try following
var arr = [
[[1],[2]],
[[4],[5]],
[[7],[8]]
];
var other = JSON.parse(JSON.stringify(arr));
other[0][0][0] = "Uh Oh";
console.log(arr);
Reasoning
arr.slice() creates different reference for the array, however, any child object/array will still continue to hold the same reference. But you need to change the references of the child object/array as well, for that convert it into a string and then back to object - It will create different references for all the objects.

How do I add the value of an array to an object

I've got code which splits the key and value at the = sign at end if line, the problem is that I have lines which have more then one =
For example:
user = aaaa
userb = bbbb
userc = test
userd = foo = bar = test
I read file with node fs and I divide it to the key and value at the = so userd causes a problem.
I am using var array = line.split('=');
Then I assign it to object in the loop
myobj = {}
myobj[array[0]] = array[1];
This is working (my obj in the loop contains all the users) until I get to userD.
So how can I concatenate all the values of userD to myObj

If we assume that everything after the first = sign is the value, regardless of whether theres another = sign then you can do this:
myobj[array[0]] = array.slice(1).join(' = ');
Now, if you want to store it simple as an array of equal values, you can do:
myobj[array[0]] = array.slice(1);
The slice function returns the same array starting at the element passed (it does more but thats what it's for here).
Update I didn't think of this answer but it's even a bit faster!

Have a look at Array.prototype.shift():
myobj[array.shift()] = array;
Unlike Array.prototype.splice(), shift() won't create an entirely new array in order to wrap up the one value we're trying to extract. See my comment here.
Here are the results from a performance test; it shows operations per second (higher is better):
UserAgent shift slice splice
Chrome 44.0.2403 4,290,938 3,993,798 1,042,423
Chrome 46.0.2468 3,631,277 4,020,712 1,098,866
Firefox 38.0 1,706,068 1,419,219 895,714
Firefox 40.0 1,739,110 1,508,623 955,794
IE 11 in Compatibility Mode 10.0.0 3,393,698 2,855,417 1,721,229
Total 14,761,091 13,797,769 5,714,026
According to these results, shift() is the fastest with slice() right behind. splice(), on the other hand, is a good bit slower than both... mainly because it's ill-suited to this particular task.
That said, I was surprised. I expected this answer to be the fastest.

Use splice function:
myobj[array.splice(0, 1)] = array;
This will cut the first item out of your array and return it as myobj parameter, the rest is going to be kept in the array, so you can either use array as an object property or let's say join it to a string:
myobj[array.splice(0, 1)] = array.join(", ");

Array.prototype.map() and Array.prototype.forEach()

I've an array (example array below) -
a = [{"name":"age","value":31},
{"name":"height (inches)","value":62},
{"name":"location","value":"Boston, MA"},
{"name":"gender","value":"male"}];
I want to iterate through this array of objects and produce a new Object (not specifically reduce).
I've these two approaches -
a = [{"name":"age","value":31},
{"name":"height (inches)","value":62},
{"name":"location","value":"Boston, MA"},
{"name":"gender","value":"male"}];
// using Array.prototype.map()
b = a.map(function(item){
var res = {};
res[item.name] = item.value;
return res;
});
console.log(JSON.stringify(b));
var newObj = [];
// using Array.prototype.forEach()
a.forEach(function(d){
var obj = {};
obj[d.name] = d.value;
newObj.push(obj)
});
console.log(JSON.stringify(newObj))
Is it not right to just use either one for this sort of operations?
Also, I'd like to understand the use case scenarios where one will be preferred over the other? Or should I just stick to for-loop?

As you've already discussed in the comments, there's no outright wrong answer here. Aside from some rather fine points of performance, this is a style question. The problem you are solving can be solved with a for loop, .forEach(), .reduce(), or .map().
I list them in that order deliberately, because each one of them could be re-implemented using anything earlier in the list. You can use .reduce() to duplicate .map(), for instance, but not the reverse.
In your particular case, unless micro-optimizations are vital to your domain, I'd make the decision on the basis of readability and code-maintenance. On that basis, .map() does specifically and precisely what you're after; someone reading your code will see it and know you're consuming an array to produce another array. You could accomplish that with .forEach() or .reduce(), but because those are capable of being used for more things, someone has to take that extra moment to understand what you ARE using them for. .map() is the call that's most expressive of your intent.
(Yes, that means in essence prioritizing efficiency-of-understanding over efficiency-of-execution. If the code isn't part of a performance bottleneck in a high-demand application, I think that's appropriate.)
You asked about scenarios where another might be preferred. In this case, .map() works because you're outputting an array, and your output array has the same length as your input array. (Again; that's what .map() does). If you wanted to output an array, but you might need to produce two (or zero) elements of output for a single element of input, .map() would be out and I'd probably use .reduce(). (Chaining .filter().map() would also be a possibility for the 'skip some input elements' case, and would be pretty legible)
If you wanted to split the contents of the input array into multiple output arrays, you could do that with .reduce() (by encapsulating all of them as properties of a single object), but .forEach() or the for loop would look more natural to me.

First, either of those will work and with your example there's no reason not to use which ever is more comfortable for your development cycle. I would probably use map since that is what is for; to create "a new array with the results of calling a provided function on every element in this array."
However, are you asking which is the absolute fastest? Then neither of those; the fastest by 2.5-3x will be a simple for-loop (see http://jsperf.com/loop-vs-map-vs-foreach for a simple comparison):
var newObj = [];
for (var i = 0, item; item = a[i]; i++) {
var obj = {};
obj[item.name] = item.value;
newObj.push(obj);
});
console.log(JSON.stringify(newObj));

How to compare two objects

I have a scenario, where I have two different Objects.
Scenario to achieve:
From two objects I need to match the values which has "A1","B2", etc...
Since both the objects values are not in proper order, the loop is breaking and missing some values.
In my demo the object1 has same repeated value i.e. "C3", It should be displayed only once.
Final output required is I need to detect only the matched values from two objects and display its corresponding "a" and "b values."
I have tried almost 90%, but somewhere some minor error is breaking my loop, Please help me out.
Sample code:
for(var i=0;i<obj1.results[0].loc.length;i++){
var findA = obj1.results[0].loc[i].anc[0].title;
for(var j=0;j< obj2.ILoc.length;j++){
var findB = obj2.ILoc[j].ais;
if(findA == findB) {
var a = obj1.results[0].loc[i].a;
var b = obj1.results[0].loc[i].b;
console.log(a);
console.log(b);
}
}
}
This is what I have tried:
Demo Link

I would recommend using for...in loop, since you're using objects instead of arrays.
for (variable in object) {...
}
If length property of both objects is equal, then this kind of loop alone will help you to compare objects with ease.

I would recommend using the diff module. You can use it in node.js and the browser.

Most efficient way to convert an HTMLCollection to an Array

Is there a more efficient way to convert an HTMLCollection to an Array, other than iterating through the contents of said collection and manually pushing each item into an array?

var arr = Array.prototype.slice.call( htmlCollection )
will have the same effect using "native" code.
Edit
Since this gets a lot of views, note (per #oriol's comment) that the following more concise expression is effectively equivalent:
var arr = [].slice.call(htmlCollection);
But note per #JussiR's comment, that unlike the "verbose" form, it does create an empty, unused, and indeed unusable array instance in the process. What compilers do about this is outside the programmer's ken.
Edit
Since ECMAScript 2015 (ES 6) there is also Array.from:
var arr = Array.from(htmlCollection);
Edit
ECMAScript 2015 also provides the spread operator, which is functionally equivalent to Array.from (although note that Array.from supports a mapping function as the second argument).
var arr = [...htmlCollection];
I've confirmed that both of the above work on NodeList.
A performance comparison for the mentioned methods: http://jsben.ch/h2IFA

not sure if this is the most efficient, but a concise ES6 syntax might be:
let arry = [...htmlCollection]
Edit: Another one, from Chris_F comment:
let arry = Array.from(htmlCollection)

I saw a more concise method of getting Array.prototype methods in general that works just as well. Converting an HTMLCollection object into an Array object is demonstrated below:
[].slice.call( yourHTMLCollectionObject );
And, as mentioned in the comments, for old browsers such as IE7 and earlier, you simply have to use a compatibility function, like:
function toArray(x) {
for(var i = 0, a = []; i < x.length; i++)
a.push(x[i]);
return a
}
I know this is an old question, but I felt the accepted answer was a little incomplete; so I thought I'd throw this out there FWIW.

For a cross browser implementation I'd sugguest you look at prototype.js $A function
copyed from 1.6.1:
function $A(iterable) {
if (!iterable) return [];
if ('toArray' in Object(iterable)) return iterable.toArray();
var length = iterable.length || 0, results = new Array(length);
while (length--) results[length] = iterable[length];
return results;
}
It doesn't use Array.prototype.slice probably because it isn't available on every browser. I'm afraid the performance is pretty bad as there a the fall back is a javascript loop over the iterable.

This works in all browsers including earlier IE versions.
var arr = [];
[].push.apply(arr, htmlCollection);
Since jsperf is still down at the moment, here is a jsfiddle that compares the performance of different methods. https://jsfiddle.net/qw9qf48j/

To convert array-like to array in efficient way we can make use of the jQuery makeArray :
makeArray: Convert an array-like object into a true JavaScript array.
Usage:
var domArray = jQuery.makeArray(htmlCollection);
A little extra:
If you do not want to keep reference to the array object (most of the time HTMLCollections are dynamically changes so its better to copy them into another array, This example pay close attention to performance:
var domDataLength = domData.length //Better performance, no need to calculate every iteration the domArray length
var resultArray = new Array(domDataLength) // Since we know the length its improves the performance to declare the result array from the beginning.
for (var i = 0 ; i < domDataLength ; i++) {
resultArray[i] = domArray[i]; //Since we already declared the resultArray we can not make use of the more expensive push method.
}
What is array-like?
HTMLCollection is an "array-like" object, the array-like objects are similar to array's object but missing a lot of its functionally definition:
Array-like objects look like arrays. They have various numbered
elements and a length property. But that’s where the similarity stops.
Array-like objects do not have any of Array’s functions, and for-in
loops don’t even work!

This is my personal solution, based on the information here (this thread):
var Divs = new Array();
var Elemns = document.getElementsByClassName("divisao");
try {
Divs = Elemns.prototype.slice.call(Elemns);
} catch(e) {
Divs = $A(Elemns);
}
Where $A was described by Gareth Davis in his post:
function $A(iterable) {
if (!iterable) return [];
if ('toArray' in Object(iterable)) return iterable.toArray();
var length = iterable.length || 0, results = new Array(length);
while (length--) results[length] = iterable[length];
return results;
}
If browser supports the best way, ok, otherwise will use the cross browser.

I suppose that calling Array.prototype functions on instances of HTMLCollection is a much better option than converting collections to arrays (e.g.,[...collection] or Array.from(collection)), because in the latter case a collection is unnecessarily implicitly iterated and a new array object is created, and this eats up additional resources. Array.prototype iterating functions can be safely called upon objects with consecutive numeric keys starting from [0] and a length property with a valid number value of such keys' quantity (including, e.g., instances of HTMLCollection and FileList), so it's a reliable way. Also, if there is a frequent need in such operations, an empty array [] can be used for quick access to Array.prototype functions; or a shortcut for Array.prototype can be created instead. A runnable example:
const _ = Array.prototype;
const collection = document.getElementById('ol').children;
alert(_.reduce.call(collection, (acc, { textContent }, i) => {
return acc += `${i+1}) ${textContent}` + '\n';
}, ''));
<ol id="ol">
<li>foo</li>
<li>bar</li>
<li>bat</li>
<li>baz</li>
</ol>

Sometimes, Even You have written code the correct way, But still it doesn't work properly.
var allbuttons = document.getElementsByTagName("button");
console.log(allbuttons);
var copyAllButtons = [];
for (let i = 0; i < allbuttons.length; i++) {
copyAllButtons.push(allbuttons[i]);
}
console.log(copyAllButtons);
you get empty array.
Like, This
HTMLCollection []
[]
Console_javascript
For Solving this problem, You have to add link of javascript file after body tag in html file.
<script src="./script.js"></script>
As you can see below,
html_file
Final Output
HTMLCollection(6) [button.btn.btn-dark.click-me, button.btn.btn-dark.reset, button#b, button#b, button#b, button#b, b: button#b]
(6) [button.btn.btn-dark.click-me, button.btn.btn-dark.reset, button#b, button#b, button#b, button#b]