Develop Reference

JavaScript - Issues recovering a map in an object after being saved in localStorage

I've been dealing with this for some time. I've a list of sections in which the user checks some checkboxes and that is sent to the server via AJAX. However, since the user can return to previous sections, I'm using some objects of mine to store some things the user has done (if he/she already finished working in that section, which checkboxes checked, etc). I'm doing this to not overload the database and only send new requests to store information if the user effectively changes a previous checkbox, not if he just starts clicking "Save" randomly. I'm using objects to see the sections of the page, and storing the previous state of the checkboxes in a Map. Here's my "supervisor":
function Supervisor(id) {
this.id = id;
this.verif = null;
this.selections = new Map();
var children = $("#ContentPlaceHolder1_checkboxes_div_" + id).children().length;
for (var i = 0; i < children; i++) {
if (i % 2 == 0) {
var checkbox = $("#ContentPlaceHolder1_checkboxes_div_" + id).children()[i];
var idCheck = checkbox.id.split("_")[2];
this.selections.set(idCheck, false);
}
}
console.log("Length " + this.selections.size);
this.change = false;
}
The console.log gives me the expected output, so I assume my Map is created and initialized correctly. Since the session of the user can expire before he finishes his work, or he can close his browser by accident, I'm storing this object using local storage, so I can change the page accordingly to what he has done should anything happen. Here are my functions:
function setObj(id, supervisor) {
localStorage.setItem(id, JSON.stringify(supervisor));
}
function getObj(key) {
var supervisor = JSON.parse(localStorage.getItem(key));
return supervisor;
}
So, I'm trying to add to the record whenever an user clicks in a checkbox. And this is where the problem happens. Here's the function:
function checkboxClicked(idCbx) {
var idSection = $("#ContentPlaceHolder1_hdnActualField").val();
var supervisor = getObj(idSection);
console.log(typeof (supervisor)); //Returns object, everythings fine
console.log(typeof (supervisor.change)); //Returns boolean
supervisor.change = true;
var idCheck = idCbx.split("_")[2]; //I just want a part of the name
console.log(typeof(supervisor.selections)); //Prints object
console.log("Length " + supervisor.selections.size); //Undefined!
supervisor.selections.set(idCheck, true); //Error! Note: The true is just for testing purposes
setObj(idSection, supervisor);
}
What am I doing wrong? Thanks!

Please look at this example, I removed the jquery id discovery for clarity. You'll need to adapt this to meet your needs but it should get you mostly there.
const mapToJSON = (map) => [...map];
const mapFromJSON = (json) => new Map(json);
function Supervisor(id) {
this.id = id;
this.verif = null;
this.selections = new Map();
this.change = false;
this.selections.set('blah', 'hello');
}
Supervisor.from = function (data) {
const id = data.id;
const supervisor = new Supervisor(id);
supervisor.verif = data.verif;
supervisor.selections = new Map(data.selections);
return supervisor;
};
Supervisor.prototype.toJSON = function() {
return {
id: this.id,
verif: this.verif,
selections: mapToJSON(this.selections)
}
}
const expected = new Supervisor(1);
console.log(expected);
const json = JSON.stringify(expected);
const actual = Supervisor.from(JSON.parse(json));
console.log(actual);
If you cant use the spread operation in 'mapToJSON' you could loop and push.
const mapToJSON = (map) => {
const result = [];
for (let entry of map.entries()) {
result.push(entry);
}
return result;
}
Really the only thing id change is have the constructor do less, just accept values, assign with minimal fiddling, and have a factory query the dom and populate the constructor with values. Maybe something like fromDOM() or something. This will make Supervisor more flexible and easier to test.
function Supervisor(options) {
this.id = options.id;
this.verif = null;
this.selections = options.selections || new Map();
this.change = false;
}
Supervisor.fromDOM = function(id) {
const selections = new Map();
const children = $("#ContentPlaceHolder1_checkboxes_div_" + id).children();
for (var i = 0; i < children.length; i++) {
if (i % 2 == 0) {
var checkbox = children[i];
var idCheck = checkbox.id.split("_")[2];
selections.set(idCheck, false);
}
}
return new Supervisor({ id: id, selections: selections });
};
console.log(Supervisor.fromDOM(2));
You can keep going and have another method that tries to parse a Supervisor from localStorageand default to the dom based factory if the localStorage one returns null.

How would I use local storage for a to do list?

I am being asked to have a to do list and save each task (that the user supplies as well as original) through local storage. My teacher did a very simple demo on something completely different and I spent a few hours trying to figure it out. When I looked at the solution, I honestly cannot figure it out. It looks really complicated, and I don't even know where to start. If anyone can give me any hints, that would be awesome!
My code:
let ul = document.querySelector('ul');
let newItem = document.querySelector('input[type=text]');
let checkbox = document.createElement('input');
checkbox.setAttribute('type', 'checkbox');
function output() {
let newTodo = document.createElement('li');
newTodo.innerText = newItem.value;
newTodo.classList.add('todo');
let ulAppend = ul.append(newTodo);
ul.append(newTodo);
let checkboxAppend = newTodo.append(checkbox);
newTodo.append(checkbox);
newItem.value = '';
}
let button = document.querySelector('.btn');
button.addEventListener('click', output);
ul.addEventListener('click', function(e) {
if (e.target.tagName === 'LI') {
e.target.remove();
} else if (e.target.tagName === 'INPUT') {
e.target.parentElement.classList.toggle('finished');
}
});
My teacher's code/local storage solution:
const todoForm = document.getElementById("newTodoForm");
const todoList = document.getElementById("todoList");
// retrieve from localStorage
const savedTodos = JSON.parse(localStorage.getItem("todos")) || [];
for (let i = 0; i < savedTodos.length; i++) {
let newTodo = document.createElement("li");
newTodo.innerText = savedTodos[i].task;
newTodo.isCompleted = savedTodos[i].isCompleted ? true : false;
if (newTodo.isCompleted) {
newTodo.style.textDecoration = "line-through";
}
todoList.appendChild(newTodo);
}
todoForm.addEventListener("submit", function(event) {
event.preventDefault();
let newTodo = document.createElement("li");
let taskValue = document.getElementById("task").value;
newTodo.innerText = taskValue;
newTodo.isCompleted = false;
todoForm.reset();
todoList.appendChild(newTodo);
// save to localStorage
savedTodos.push({ task: newTodo.innerText, isCompleted: false });
localStorage.setItem("todos", JSON.stringify(savedTodos));
});
todoList.addEventListener("click", function(event) {
let clickedListItem = event.target;
if (!clickedListItem.isCompleted) {
clickedListItem.style.textDecoration = "line-through";
clickedListItem.isCompleted = true;
} else {
clickedListItem.style.textDecoration = "none";
clickedListItem.isCompleted = false;
}
// breaks for duplicates - another option is to have dynamic IDs
for (let i = 0; i < savedTodos.length; i++) {
if (savedTodos[i].task === clickedListItem.innerText) {
savedTodos[i].isCompleted = clickedListItem.isCompleted;
localStorage.setItem("todos", JSON.stringify(savedTodos));
}
}
});
Even though my code is more simpler (at least from what I can tell), it works exactly as his code does.

Local storage saves a JSON object to the user's computer. You should create an array of todos, append that array with every new todo, then set that item to local storage.
let ul = document.querySelector('ul');
const savedTodos = JSON.parse(localStorage.getItem("todos")) || []; // Retrieves local storage todo OR creates empty array if none exist
let newItem = document.querySelector('input[type=text]');
let checkbox = document.createElement('input');
checkbox.setAttribute('type', 'checkbox');
function output() {
let newTodo = document.createElement('li');
newTodo.innerText = newItem.value;
newTodo.classList.add('todo');
ul.append(newTodo);
newTodo.append(checkbox);
savedTodos.push({task: newItem.value, isCompleted: false}); // Appends the new todo to array
localStorage.setItem("todos", JSON.stringify(savedTodos)); //Converts object to string and stores in local storage
newItem.value = '';
}

I've annotated the solution you posted with some comments to help you step through it.
// Retrieve elements and store them in variables
const todoForm = document.getElementById("newTodoForm");
const todoList = document.getElementById("todoList");
// Get data stored in localStorage under the key "todos".
// The data type will be a string (local storage can only store strings).
// JSON is a global object that contains methods for working with data represented as strings.
// The `||` syntax is an OR operator and is used here to set an empty array as a fallback in case `localStorage` is empty
const savedTodos = JSON.parse(localStorage.getItem("todos")) || [];
// Create a loop the same length as the list of todos
for (let i = 0; i < savedTodos.length; i++) {
// Create an <li> element in memory (does not appear in the document yet)
let newTodo = document.createElement("li");
// Set the inner text of that new li with the contents from local storage.
// The savedTodos[i] is accessing data in the localStorage array.
// The [i] is a different number each loop.
// The `.task` is accessing 'task' property on the object in the array.
newTodo.innerText = savedTodos[i].task;
// Create a new property on the element called `isCompleted` and assign a boolean value.
// This is only accessible in code and will not show up when appending to the DOM.
newTodo.isCompleted = savedTodos[i].isCompleted ? true : false;
// Check the value we just set.
if (newTodo.isCompleted) {
// Create a style for the element if it is done (strike it out)
newTodo.style.textDecoration = "line-through";
}
// Actually append the new element to the document (this will make it visible)
todoList.appendChild(newTodo);
}
// `addEventListener` is a function that registers some actions to take when an event occurs.
// The following tells the browser - whenever a form is submitted, run this function.
todoForm.addEventListener("submit", function(event) {
// Don't try to send the form data to a server. Stops page reloading.
event.preventDefault();
// Create a <li> element in memory (not yet visible in the document)
let newTodo = document.createElement("li");
// Find element in the document (probably a input element?) and access the text value.
let taskValue = document.getElementById("task").value;
// Set the text of the <li>
newTodo.innerText = taskValue;
// Set a property on the <li> call `isCompleted`
newTodo.isCompleted = false;
// Empty out all the input fields in the form
todoForm.reset();
// Make the new <li> visible in the document by attaching it to the list
todoList.appendChild(newTodo);
// `push` adds a new element to the `savedTodos` array. In this case, an object with 2 properties.
savedTodos.push({ task: newTodo.innerText, isCompleted: false });
// Overwrite the `todos` key in local storage with the updated array.
// Use the JSON global object to turn an array into a string version of the data
// eg [1,2,3] becomes "[1,2,3]"
localStorage.setItem("todos", JSON.stringify(savedTodos));
});
// This tells the browser - whenever the todoList is clicked, run this function.
// The browser will call the your function with an object that has data about the event.
todoList.addEventListener("click", function(event) {
// the `target` of the event is the element that was clicked.
let clickedListItem = event.target;
// If that element has a property called `isCompleted` set to true
if (!clickedListItem.isCompleted) {
// update the styles and toggle the `isCompleted` property.
clickedListItem.style.textDecoration = "line-through";
clickedListItem.isCompleted = true;
} else {
clickedListItem.style.textDecoration = "none";
clickedListItem.isCompleted = false;
}
// The code above changes the documents version of the data (the elements themselves)
// This loop ensures that the array of todos data is kept in sync with the document
// Loop over the array
for (let i = 0; i < savedTodos.length; i++) {
// if the item in the array has the same text as the item just clicked...
if (savedTodos[i].task === clickedListItem.innerText) {
// toggle the completed state
savedTodos[i].isCompleted = clickedListItem.isCompleted;
// Update the localStorage with the new todos array.
localStorage.setItem("todos", JSON.stringify(savedTodos));
}
}
});
Keep in mind, there are 2 sources of state in your todo list. One is how the document looks, and the other is the array of todos data. Lots of challenges come from making sure these 2 stay in sync.
If somehow the document showed one of the list items as crossed out, but your array of data shows that all the todos are not completed, which version is correct? There is no right answer here, but state management will be something you might consider when designing apps in the future. Redux is a good js library with a well understood pattern that helps solve this problem. Hope this last comment doesn't confuse too much. Best of luck!

The important part is in (de)serializing the data. That means:
reading from localStorage (JSON.parse(localStorage.getItem("todos")) || [])
We add the default [] because if the todos key does not exist, we will get null and we expect a list
saving to localStorage (localStorage.setItem("todos", JSON.stringify(savedTodos)))
We need JSON.parse and its complementary operation JSON.stringify to parse and save strings because localStorage can store only strings.
In your case you need to read the data from localStorage and render the initial list. To save it to localStorage, again, you have to serialize the data. See the below snippets (link to working JSFIDDLE, because the below example does not work in the StackOverflow sandbox environment):
let ul = document.querySelector('ul');
let newItem = document.querySelector('input[type=text]');
const Store = {
serialize () {
return [].slice.call(document.querySelectorAll("li")).map(c => {
return {
text: c.textContent,
finished: c.querySelector("input").checked
}
})
},
get () {
return JSON.parse(localStorage.getItem("todos")) || []
},
save () {
return localStorage.setItem("todos", JSON.stringify(Store.serialize()))
}
}
const firstItems = Store.get()
firstItems.forEach(it => {
output(it.text, it.finished)
})
function output(v, finished) {
let newTodo = document.createElement('li');
newTodo.innerText = v || newItem.value;
newTodo.classList.add('todo');
let ulAppend = ul.append(newTodo);
ul.append(newTodo);
// Create a checkbox for each item
let checkbox = document.createElement('input');
if (finished) {
checkbox.checked = true
}
checkbox.setAttribute('type', 'checkbox');
let checkboxAppend = newTodo.append(checkbox);
newTodo.append(checkbox);
newItem.value = '';
}
let button = document.querySelector('.btn');
button.addEventListener('click', () => {
output()
Store.save()
});
ul.addEventListener('click', function(e) {
if (e.target.tagName === 'LI') {
e.target.remove();
} else if (e.target.tagName === 'INPUT') {
e.target.parentElement.classList.toggle('finished');
}
// Update the value in localStorage when you delete or add a new item
Store.save()
});
<ul></ul>
<input type="text" /> <button class="btn">Submit</button>
I have added the Store variable to simplify the way you get and set the data in localStorage.
The serialize method will read the TODOs from the list. document.querySelectorAll("li") returns a NodeList, but by doing [].slice.call(...) we convert it to an Array.

Remove one item from an array when clicked by only js

I am new to JavaScript. I have a small code that creates list from input and then adds it to an array. I am able to remove one item from the DOM when the item is clicked, but I couldn't remove it from the array.
I tried to use array.splice(item, 1)
lists.addEventListener("click", function (e) {
e.target.closest("li").remove();
userInputArr.splice(item, 1);});
But it removes the entire array sometime, and sometime removes the last item. when I console log the code, it looks like I clicked 3 or 4 times on the list even though I just clicked once. I have no idea what's wrong. this is the entire code:
const lists = document.querySelector(".lists");
const userInput = document.querySelector(".add-note");
const addBtn = document.querySelector(".add-btn");
const item = document.querySelectorAll(".list");
userInputArr = [];
function addNote() {
if (userInput.value < 1) {
return;
}
lists.insertAdjacentHTML(
"afterbegin",
`<li class='list'>${userInput.value}</li>`
);
userInputArr.push(lists);
lists.addEventListener("click", function (e) {
e.target.closest("li").remove();
userInputArr.splice(item, 1);
});
userInput.value = "";
}
addBtn.addEventListener("click", function () {
addNote();
});

Code is totally meaningless
1)
userInputArr.push(lists)
why you push the same element all the time? As lists refers to the first and the only element with class 'lists'?
2)
userInputArr.splice(item, 1)
please watch carefully what splice does? The first argument is number, but you pass a collection of elements with class 'list'. But i camn not even suggest which element should be removed as it contains the same element as i mentioned in first point
3) You do not need this array at all
So right approach is something like this
const lists = document.querySelector(".lists");
// just once create listener, no need to do it each time
lists.addEventListener("click", function (e) {
// if you want to remove clicked item then
if (e.target.tagName === 'LI') e.target.remove();
// but if you want to remove the first one then uncomment line
// if (this.children[0]) this.children[0].remove()
});
const userInput = document.querySelector(".add-note");
const addBtn = document.querySelector(".add-btn");
///////////////////////////////////////////////////
// item is meaninglee here, so delete this line
// const item = document.querySelectorAll(".list");
//////////////////////
// array is useless too, delete this line
// userInputArr = [];
function addNote() {
// check if it is number
if (isNaN(userInput.value) || Number(userInput.value < 1)) {
return;
}
lists.insertAdjacentHTML(
"afterbegin",
`<li class='list'>${userInput.value}</li>`
);
userInput.value = "";
}
addBtn.addEventListener("click", function () {
addNote();
});

const items = (() => {
const _items = {};
let key = 0;
return {
put(value) {
_items[key++] = value;
console.log("Added", this.all());
return key - 1;
},
remove(key) {
delete _items[key++];
console.log("Removed", this.all());
},
all(asArray = true) {
return asArray ? Object.values(_items) : { ..._items
};
}
}
})();
const inputEl = document.querySelector(".input");
const itemsEl = document.querySelector(".items");
const addBtn = document.querySelector(".btn-add");
addBtn.addEventListener("click", () => {
const value = inputEl.value.trim();
if (!value.length) return;
const key = items.put(value);
const li = document.createElement("li");
li.textContent = value;
li.dataset.key = key;
itemsEl.append(li);
inputEl.value = "";
});
itemsEl.addEventListener("click", (e) => {
const li = e.target.closest("li");
items.remove(li.dataset.key);
li.remove();
});
<input type="text" class="input">
<button class="btn-add">Add</button>
<ul class="items"></ul>
Run code & View in full screen.

use shift() userInputArr.shift()
you are also getting double clicks because your addNote() function contains an event listener lists.addEventListener and it's executed by another event listner addBtn.addEventListener you should probably move
lists.addEventListener out of the addNote function

Populating array with objects from looping through checked radio buttons gives same value each time

Code below:
$(".day_a_radio").change(function()
var radio_array = [];
var radio_object = {};
$("input.day_a_radio:checked").each(function() {
radio_object["id"] = $(this).data("id");
radio_object["date"] = $(this).data("date");
radio_object["type"] = $(this).data("type");
radio_array.push(radio_object);
});
console.log(radio_array);
I have a few dozen radio buttons with data attributes of id, date and type. When one of them is clicked, I loop through them all. Then within that loop, for each one that is checked, I populate an array with objects.
However, when I do the console.log, the ID, date and type are all the same despite all the radio buttons having different values
Any ideas?

You have to make a new object, otherwise the same one is updated and pushed repeatedly.
$(".day_a_radio").change(function(){
var radio_array = [];
var radio_object = {};
$("input.day_a_radio:checked").each(function() {
// MAKE A NEW OBJECT
radio_object = {};
radio_object["id"] = $(this).data("id");
radio_object["date"] = $(this).data("date");
radio_object["type"] = $(this).data("type");
radio_array.push(radio_object);
});
You could also simplify this a little with a map.
$(".day_a_radio").change(function(){
var radio_array = $("input.day_a_radio:checked").map(function() {
var $this = $(this);
return {
id: $this.data("id"),
date: $this.data("date"),
type: $this.data("type")
};
}).get();
});

You're pushing the same reference to array every time you need to create a new copy every time and than push
$(".day_a_radio").change(function()
var radio_array = [];
$("input.day_a_radio:checked").each(function() {
var obj = {} // It creates a new object everytime
obj["id"] = $(this).data("id");
obj["date"] = $(this).data("date");
obj["type"] = $(this).data("type");
radio_array.push(obj);
});
console.log(radio_array);

How to update JavaScript array dynamically

I have an empty javascript array(matrix) that I created to achieve refresh of divs. I created a function to dynamically put data in it. Then I created a function to update the Array (which I have issues).
The Data populated in the Array are data attributes that I put in a JSON file.
To better undertand, here are my data attributes which i put in json file:
var currentAge = $(this).data("age");
var currentDate = $(this).data("date");
var currentFullName = $(this).data("fullname");
var currentIDPerson = $(this).data("idPerson");
var currentGender = $(this).data("gender");
Creation of the array:
var arrayData = [];
Here is the function a created to initiate and addind element to the Array :
function initMatrix(p_currentIDPerson, p_currentGender, p_currentFullName, p_currentDate, p_currentAge) {
var isFound = false;
// search if the unique index match the ID of the HTML one
for (var i = 0; i < arrayData.length; i++) {
if(arrayData[i].idPerson== p_currentIDPerson) {
isFound = true;
}
}
// If it doesn't exist we add elements
if(isFound == false) {
var tempArray = [
{
currentIDPerson: p_currentIDPerson,
currentGender: p_currentGender,
currentFullName: p_currentFullName,
currentDate: p_currentDate, currentAge: p_currentAge
}
];
arrayData.push(tempArray);
}
}
The update function here is what I tried, but it doesn't work, maybe I'm not coding it the right way. If you can help please.
function updateMatrix(p_currentIDPerson, p_currentGender, p_currentFullName, p_currentDate, p_currentAge) {
for (var i = 0; i < arguments.length; i++) {
for (var key in arguments[i]) {
arrayData[i] = arguments[i][key];
}
}
}
To understand the '$this' and elm: elm is the clickableDivs where I put click event:
(function( $ ) {
// Plugin to manage clickable divs
$.fn.infoClickable = function() {
this.each(function() {
var elm = $( this );
//Call init function
initMatrixRefresh(elm.attr("idPerson"), elm.data("gender"), elm.data("fullname"), elm.data("date"), elm.data("age"));
//call function update
updateMatrix("idTest", "Alarme", "none", "10-02-17 08:20", 10);
// Définition de l'evenement click
elm.on("click", function(){});
});
}
$('.clickableDiv').infoClickable();
}( jQuery ));
Thank you in advance

Well... I would recommend you to use an object in which each key is a person id for keeping this list, instead of an array. This way you can write cleaner code that achieves the same results but with improved performance. For example:
var myDataCollection = {};
function initMatrix(p_currentIDPerson, p_currentGender, p_currentFullName, p_currentDate, p_currentAge) {
if (!myDataCollection[p_currentIDPerson]) {
myDataCollection[p_currentIDPerson] = {
currentIDPerson: p_currentIDPerson,
currentGender: p_currentGender,
currentFullName: p_currentFullName,
currentDate: p_currentDate,
currentAge: p_currentAge
};
}
}
function updateMatrix(p_currentIDPerson, p_currentGender, p_currentFullName, p_currentDate, p_currentAge) {
if (myDataCollection[p_currentIDPerson]) {
myDataCollection[p_currentIDPerson] = {
currentGender: p_currentGender,
currentFullName: p_currentFullName,
currentDate: p_currentDate,
currentAge: p_currentAge
};
}
}
Depending on your business logic, you can remove the if statements and keep only one function that adds the object when there is no object with the specified id and updates the object when there is one.

I think the shape of the resulting matrix is different than you think. Specifically, the matrix after init looks like [ [ {id, ...} ] ]. Your update function isn't looping enough. It seems like you are trying to create a data structure for storing and updating a list of users. I would recommend a flat list or an object indexed by userID since thats your lookup.
var userStorage = {}
// add/update users
userStorage[id] = {id:u_id};
// list of users
var users = Object.keys(users);