Javascript: Linked List: Unable to delete object reference - javascript

I'm doing a Linked List data structure. The prototype includes a method to pop (delete) the last item from the list which I'm attempting to do by finding the last object, and then setting it to null. It does not seem to work. What does work is setting the reference (the 'pointer') in the previous object to null. I'm still a relative JS OOP newbie, can't get my brain to understand why. The code:
function LinkedList() {
this._rootNode = null;
this._length = 0;
}
LinkedList.prototype = {
push: function(data) {
var newNode = {
data: data,
nextNode: null
};
// initialize this._rootNode or subsequent .nextNode with newNode
this._length++;
},
pop: function() {
var selectedNode, perviousNode;
if ( this._rootNode ) {
if ( this._length > 1 ) {
selectedNode = this._rootNode;
while ( selectedNode.nextNode ) {
previousNode = selectedNode; // <-- shouldn't need this?
selectedNode = selectedNode.nextNode;
}
selectedNode = null; // <-- doesn't delete it
// previousNode.nextNode = null; // <-- works (but feels unnecessary?)
} else {
this._rootNode = null;
}
this._length--;
}
},
// more methods..
};
/* --- Main Prorgam --- */
var list = new LinkedList();
list.push('AAA');
list.push('BBB');
list.pop();
console.log(list._rootNode.nextNode.data); <-- 'BBB' still there
Would appreciate some insight, and any other tips on improving the function. Thanks!

I guess you realize that your push method doesn't work, but you haven't asked about that one.
If you are doing some kind of school project that requires you to write a linked list like this, then by all means, continue. Your issue is that selectedNode is not really "the node itself", it's a reference to it, and you're just setting that reference to null while the previous item's nextNode pointer still refers to it, so you haven't actually removed it from your list. You would actually do so by un-commenting the line setting that pointer to null, which means you also have to leave in the line saving the reference to the previous node.
previousNode.nextNode = null;
You actually don't want to delete the node entirely with pop(), you want to return it. Once you remove the reference to the popped node in your calling function though, it will be the last reference and the object will be made available for garbage collection. This is (to my knowledge) how all traditional OOP languages handle linked lists at the basic level.
Which brings me to my next point, that most OOP languages you'll use these days don't actually require you to work on the basic level. Most of them have libraries that will implement linked lists for you and Javascript in particular essentially implements a linked list-style data structure in its array syntax. To the point where ([1,2,3,4]).pop() evaluates to 4 and ([1,2,3,4]).push(5) evaluates to [1,2,3,4,5]. If you actually need to USE a linked list in a real project, just don't.

Related

Javascript not setting this to value with apply or call

Edit: the code below was made up on the spot to show how I was going about what I was doing. It definietely won't run, it is missing a lot of things.
Here is a working example in codepen: https://codepen.io/goducks/pen/XvgpYW
much shorter example: https://codepen.io/goducks/pen/ymXMyB
When creating a function that is using call or apply, the this value stays null when using getPerson. however, when I use apply or call with getPerson it returns the correct person.
Please critique, I am really starting to learn more and more. I am in the middle of a project section so it might be hard to change all the code, but my next project could implement this better.
call and apply are setting to the window and not the object.
I will provide code that is much simpler with the same concept of what I am talking about.
function createPerson(){
this.manager = null;
this.teamManager = null;
this.setTeamManager = function(val){
this.teamManager = val;
}
this.setManager = function(val){
console.log('setting manager to',val);
this.teamManager = val;
}
this.getTeamManager = function(){
console.log('setting team manager to',val);
return this.teamManager ;
}
this.getManager = function(){
return this.manager;
}
this.appendSelect = function(elem){
var that = this;
createOtherSelects(that,elem);
}
//some functions that create selects with managers etc
//now assume there are other selects that will filter down the teams,
//so we might have a function that creates on change events
function createOtherSelects(that){
//code that creates locations, depending on location chosen will
//filter the managers
$('#location').on('change',function(){
//do some stuff
//... then call create management
createManagement(that,elem);
});
}
function createManagement(that,elem){
var currentLocation = that.location; //works
var area = that.area;//works ... assume these are set above
//code that returns a filter and unique set of managers back
that.teamManager = [...new Set(
data.map(person=>{
if(person.area==area &&
person.currentLocation==currentLocation
){
return person;
}
})
)].filter(d=>{if(d){return d}});
if(elem.length>0){
var selectNames = ['selectManager','selectTeamManager'];
var fcns = [that.setManager,that.setTeamManager];
for(var i = 0; i < selectNames.length;i++){
//do stuff
if(certainCriteriaMet){
// filter items
if(filteredManager == 1){
fcns[i].call(null,currentManager);//
}
}
}
}
}
}
var xx = new createPerson()
In console I see setting manager and setting team manager to with the correct values.
however when I call xx in console, I see everything else set except for
xx.teamManager and xx.manager
instead it is applying to the window, so if I type teamManager in the console, it will return with the correct person.
If I straight up say
that.setManager('Steve')
or even it works just fine.
xx.setManager('steve')
the this value in setManager is somehow changing from the current instance of the object to this window. I don't know why, and I would like to learn how to use apply and call using that for future reference.
I think the issue is with your following code
fcns[i].call(null,currentManager)
If you are not supplying "this" to call, it will be replaced with global object in non-strict mode.
fcns[i].call(that,currentManager)
See mdn article here
From your codepen example, you need to change that line
fcnset[0].apply(that,[randomName]);
The first argument of the apply method is the context, if you are not giving it the context of your method it's using the global context be default. That's why you end up mutating the window object, and not the one you want !

Truly protecting data members (of classes/IIFE variables) in JavaScript

Context
I have been tasked with fixing a big bug on the menu-edit page, which was caused by a stale element issue, caused by the HTML elements for it being rendered server-side. In my three-day fight against this bug, I got some inspiration from Angular and decided to try to make a menu state that will power everything on the page (adding/removing categories/items, and later, pagination of the modals for the adding)
Some Code
I came up with this IIFE (to be the "controller" of the MVC. Selector modals hit the add methods of this, and delete buttons hit the remove methods of this. Also, this gets passed to template-render function, which is literally the first thing hit when a modal gets popped):
/* all the categories, items, and modifiers that power this page */
var menuState = (function() {
let _categories = {
attached: [],
available: []
}, _items = {
attached: [],
available: []
}, _modifiers = {
attached: [],
available: []
}
function getExposedMethodsFor(obj) {
return {
all : function() { return obj.attached.concat(obj.available) },
attached : function() { return obj.attached },
available : function() { return obj.available }
// ... other methods that work on obj.attached,obj.available
}
}
let categoryExposedMethods = getExposedMethodsFor(_categories)
// other exposer objects
return {
getAllCategories : categoryExposedMethods.all,
getAttachedCategories : categoryExposedMethods.attached,
getAvailableCategories : categoryExposedMethods.available
// the rest of the exposed methods irrelevant to this question at hand
}
})()
OK, so what's the problem?
The problem is that this is false sense of security, it seems. When I try to XSS-test this structure alone, it fails.
I test it with three entities in _categories, all of which are attached, causing
menuState.getAllCategories().length
to return 3 and
menuState.getAvailableCategories().length
to return 0. Good news is that when I tried
menuState.getAllCategories().push('a')
menuState.getAllCategories().length
I still get three.
However, when I go
menuState.getAvailableCategories().push('b')
menuState.getAvailableCategories().length
I get 1, instead of 0 !!
Is there truly a way to lock down the other getters here?! If not, what are my alternatives?
I fixed it with Object.freeze, which I already used for refactoring the "enums" the dev before me wrote when he was working on this project. What it does is fully protect a state from any type of changes, including:
adding properties
deleting properties
modifying properties
re-assigning the object/array being "frozen"
How I use it
In the helper method, I did the following :
attached : function() { return Object.freeze(obj.attached) },
available : function() { return Object.freeze(obj.available) },
This prevents the arrays being changed from those methods, thus shutting down this type of XSS. Also, menuState was declared with const.

How to set the order elements are appended to a documentFragment when being returned async?

Odd question I guess...
I'm building my DOM in memory and make heavy use of promises. Say I have this inside a for... loop:
target = document.createDocumentFragment();
promises = [], pass, skip, store;
for (i = 0; i < foo; i += 1) {
element = foo[i];
// set promise
promises[i] = app.setContent(element, {}, update)
.then(function(response) {
// HELP!
if (pass) {
target.appendChild(store)
store = undefined;
skip = undefined;
pass = undefined;
}
if (response.tagName !== undefined) {
pass = true;
}
if (skip === undefined && response.tagName === undefined) {
store = response;
skip = true;
} else {
target.appendChild(response);
}
});
RSVP.all(promises).then(continue...
The loop above runs 3x returning two div tags and a documentFragment. Without promises, no problem, my structure is
<div toolbar>
<fragment = form>
<div toolbar>
Problem is, when I converted to async I have no more say in the order items get appended forcing me to stupid things as above, where I'm finding my fragment, store it in store, set a skip parameter, append my first div, which sets pass, which will allow my stored fragment to be appended when the next promises "comes in"... what a waste of code...
Question:
Is there any way to properly set the order of items in a generic way when all items are being returned async? If not, how do I move items? Since I'm working in memory, I can select using querySelector/qsAll but I don't have a lot of other options? Any ideas?
(And please don't suggest to put in the DOM and then shuffle :-))
Thanks!
I'll explain in words ...
On making each async request, also create a container, in the right place, for the requested data to go when it eventually arrives. This container will typically be a <div> or a <span>.
Keep a reference to the container (typically in a closure) such that the async response handler knows which container corresponds to which data.

Advice needed... Javascript OOP/namespacing

right now i am at a point where i feel that i need to improve my javascript skills because i already see that what i want to realize will get quite complex. I've iterrated over the same fragment of code now 4 times and i am still not sure if it's the best way.
The task:
A user of a webpage can add different forms to a webpage which i call modules. Each form provides different user inputs and needs to be handled differently. Forms/Modules of the same type can be added to the list of forms as the user likes.
My current solution:
To make the code more readable and seperate functions i use namespaced objects. The first object holds general tasks and refers to the individual forms via a map which holds several arrays where each contains the id of a form and the reference to the object which holds all the functions which need to be performed especially for that kind of form.
The structure looks more or less similar to this:
var module_handler = {
_map : [], /* Map {reference_to_obj, id} */
init: function(){
var module = example_module; /* Predefined for this example */
this.create(module);
},
create: function(module) {
//Store reference to obj id in map
this._map.push([module,id = this.createID()]);
module.create(id);
},
createID: function(id) {
//Recursive function to find an available id
},
remove: function(id) {
//Remove from map
var idx = this._map.indexOf(id);
if(idx!=-1) this._map.splice(idx, 1);
//Remove from DOM
$('#'+id+'').remove();
}
}
var example_module = {
create: function(id) {
//Insert html
$('#'+id+' > .module_edit_inner').replaceWith("<some html>");
}
}
Now comes my question ;-)
Is the idea with the map needed?
I mean: Isn't there something more elegant like:
var moduleXYZ = new example_module(id)
which copies the object and refers only to that form.... Something more logical and making speed improvements?? The main issue is that right now i need to traverse the DOM each time if i call for example "example_module.create() or later on any other function. With this structure i cant refer to the form like with something like "this"???
Do you see any improvements at this point??? This would help me very much!!! Really i am just scared to go the wrong way now looking at all the stuff i will put on top of this ;-)
Thank You!
I think you're looking for prototype:
=========
function exampleModule(id)
{
this.id = id;
}
exampleModule.prototype.create = function()
{
}
=========
var module1 = new exampleModule(123);
module1.create();
var module2 = new exampleModule(456);
module2.create();

Dojo extending dojo.dnd.Source, move not happening. Ideas?

NOTICE: THIS IS SOLVED, I WILL PUBLISH THE SOLUTION HERE ASAP.
Hey all,
Ok... I have a simple dojo page with the bare essentials. Three UL's with some LI's in them. The idea si to allow drag-n-drop among them but if any UL goes empty due to the last item being dragged out, I will put up a message to the user to gie them some instructions.
In order to do that, I wanted to extend the dojo.dnd.Source dijit and add some intelligence. It seemed easy enough. To keep things simple (I am loading Dojo from a CDN) I am simply declating my extension as opposed to doing full on module load. The declaration function is here...
function declare_mockupSmartDndUl(){
dojo.require("dojo.dnd.Source");
dojo.provide("mockup.SmartDndUl");
dojo.declare("mockup.SmartDndUl", dojo.dnd.Source, {
markupFactory: function(params, node){
//params._skipStartup = true;
return new mockup.SmartDndUl(node, params);
},
onDndDrop: function(source, nodes, copy){
console.debug('onDndDrop!');
if(this == source){
// reordering items
console.debug('moving items from us');
// DO SOMETHING HERE
}else{
// moving items to us
console.debug('moving items to us');
// DO SOMETHING HERE
}
console.debug('this = ' + this );
console.debug('source = ' + source );
console.debug('nodes = ' + nodes);
console.debug('copy = ' + copy);
return dojo.dnd.Source.prototype.onDndDrop.call(this, source, nodes, copy);
}
});
}
I have a init function to use this to decorate the lists...
dojo.addOnLoad(function(){
declare_mockupSmartDndUl();
if(dojo.byId('list1')){
//new mockup.SmartDndUl(dojo.byId('list1'));
new dojo.dnd.Source(dojo.byId('list1'));
}
if(dojo.byId('list2')){
new mockup.SmartDndUl(dojo.byId('list2'));
//new dojo.dnd.Source(dojo.byId('list2'));
}
if(dojo.byId('list3')){
new mockup.SmartDndUl(dojo.byId('list3'));
//new dojo.dnd.Source(dojo.byId('list3'));
}
});
It is fine as far as it goes, you will notice I left "list1" as a standard dojo dnd source for testing.
The problem is this - list1 will happily accept items from lists 2 & 3 who will move or copy as apprriate. However lists 2 & 3 refuce to accept items from list1. It is as if the DND operation is being cancelled, but the debugger does show the dojo.dnd.Source.prototype.onDndDrop.call happening, and the paramaters do look ok to me.
Now, the documentation here is really weak, so the example I took some of this from may be way out of date (I am using 1.4).
Can anyone fill me in on what might be the issue with my extension dijit?
Thanks!
If you use Dojo XD loader (used with CDNs), all dojo.require() are asynchronous. Yet declare_mockupSmartDndUl() assumes that as soon as it requires dojo.dnd.Source it is available. Generally it is not guaranteed.
Another nitpicking: dojo.dnd.Source is not a widget/dijit, while it is scriptable and can be used with the Dojo Markup, it doesn't implement any Dijit's interfaces.
Now the problem — the method you are overriding has following definition in 1.4:
onDndDrop: function(source, nodes, copy, target){
// summary:
// topic event processor for /dnd/drop, called to finish the DnD operation
// source: Object
// the source which provides items
// nodes: Array
// the list of transferred items
// copy: Boolean
// copy items, if true, move items otherwise
// target: Object
// the target which accepts items
if(this == target){
// this one is for us => move nodes!
this.onDrop(source, nodes, copy);
}
this.onDndCancel();
},
Notice that it has 4 arguments, not 3. As you can see if you do not pass the 4th argument, onDrop is never going to be called by the parent method.
Fix these two problems and most probably you'll get what you want.
In the end, I hit the Dojo IRC (great folks!) and we ended up (so far) with this...
function declare_mockupSmartDndUl(){
dojo.require("dojo.dnd.Source");
dojo.provide("mockup.SmartDndUl");
dojo.declare("mockup.SmartDndUl", dojo.dnd.Source, {
markupFactory: function(params, node){
//params._skipStartup = true;
return new mockup.SmartDndUl(node, params);
},
onDropExternal: function(source, nodes, copy){
console.debug('onDropExternal called...');
// dojo.destroy(this.getAllNodes().query(".dndInstructions"));
this.inherited(arguments);
var x = source.getAllNodes().length;
if( x == 0 ){
newnode = document.createElement('li');
newnode.innerHTML = "You can drag stuff here!";
dojo.addClass(newnode,"dndInstructions");
source.node.appendChild(newnode);
}
return true;
// return dojo.dnd.Source.prototype.onDropExternal.call(this, source, nodes, copy);
}
});
}
And you can see where I am heading, I put in a message when the source is empty (client specs, ug!) and I need to find a way to kill it when something gets dragged in (since it is not, by definition, empty any more ona incomming drag!). That part isnt workign so well.
Anyway, the magic was not to use the onDnd_____ functions, but the higher level one and then call this.inherited(arguments) to fire off the built in functionality.
Thanks!
dojo.require("dojo.dnd.Source");
dojo.provide("mockup.SmartDndUl");
dojo.declare("mockup.SmartDndUl", dojo.dnd.Source, {
Dojo require statement and declare statement are next to next. I think that will cause dependencies problem.
the dojo require statement should go outside onload block and the declare statement should be in onload block.

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