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Questions tagged [javascript]

For questions about programming in ECMAScript (JavaScript/JS) and its different dialects/implementations (except for ActionScript). Note that JavaScript is NOT Java. Include all tags that are relevant to your question: e.g., [node.js], [jQuery], [JSON], [ReactJS], [angular], [ember.js], [vue.js], [typescript], [svelte], etc.

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11979 votes
155 answers
12.8m views

How can I remove a specific item from an array in JavaScript?

How do I remove a specific value from an array? Something like: array.remove(value); Constraints: I have to use core JavaScript. Frameworks are not allowed.
Walker's user avatar
  • 133k
8697 votes
67 answers
3.2m views

How do I check if an element is hidden in jQuery?

How do I toggle the visibility of an element using .hide(), .show(), or .toggle()? How do I test if an element is visible or hidden?
Philip Morton's user avatar
8493 votes
32 answers
1.2m views

What does "use strict" do in JavaScript, and what is the reasoning behind it?

Recently, I ran some of my JavaScript code through Crockford's JSLint, and it gave the following error: Problem at line 1 character 1: Missing "use strict" statement. Doing some searching, ...
Mark Rogers's user avatar
  • 97.3k
7698 votes
58 answers
7.9m views

How do I redirect to another webpage?

How can I redirect the user from one page to another using jQuery or pure JavaScript?
venkatachalam's user avatar
7637 votes
42 answers
1.2m views

var functionName = function() {} vs function functionName() {}

I've recently started maintaining someone else's JavaScript code. I'm fixing bugs, adding features and also trying to tidy up the code and make it more consistent. The previous developer used two ways ...
Richard Garside's user avatar
7615 votes
86 answers
1.6m views

How do JavaScript closures work?

How would you explain JavaScript closures to someone with a knowledge of the concepts they consist of (for example functions, variables and the like), but does not understand closures themselves? I ...
7515 votes
39 answers
3.2m views

How do I remove a property from a JavaScript object?

Given an object: let myObject = { "ircEvent": "PRIVMSG", "method": "newURI", "regex": "^http://.*" }; How do I remove the property ...
johnstok's user avatar
  • 97.7k
7406 votes
3 answers
8.1m views

How to check whether a string contains a substring in JavaScript?

Usually I would expect a String.contains() method, but there doesn't seem to be one. What is a reasonable way to check for this?
6708 votes
42 answers
2.1m views

How do I return the response from an asynchronous call?

How do I return the response/result from a function foo that makes an asynchronous request? I am trying to return the value from the callback, as well as assigning the result to a local variable ...
Felix Kling's user avatar
6331 votes
72 answers
4.4m views

How do I include a JavaScript file in another JavaScript file?

How do I include a JavaScript file inside another JavaScript file, similar to @import in CSS?
Alec Smart's user avatar
  • 95.3k
6289 votes
38 answers
2.3m views

What is the difference between "let" and "var"?

ECMAScript 6 introduced the let statement. I've heard that it's described as a local variable, but I'm still not quite sure how it behaves differently than the var keyword. What are the differences? ...
TM.'s user avatar
  • 110k
5721 votes
41 answers
5.3m views

Loop (for each) over an array in JavaScript

How can I loop through all the entries in an array using JavaScript?
Dante1986's user avatar
  • 59.4k
5645 votes
47 answers
2.2m views

Which equals operator (== vs ===) should be used in JavaScript comparisons?

I'm using JSLint to go through JavaScript, and it's returning many suggestions to replace == (two equals signs) with === (three equals signs) when doing things like comparing idSele_UNVEHtype.value....
bcasp's user avatar
  • 57.4k
5473 votes
50 answers
4.6m views

How do I replace all occurrences of a string in JavaScript?

Given a string: string = "Test abc test test abc test test test abc test test abc"; This seems to only remove the first occurrence of abc in the string above: string = string.replace('abc', ...
Ali's user avatar
  • 266k
5471 votes
79 answers
4.8m views

How can I validate an email address in JavaScript?

I'd like to check if the user input is an email address in JavaScript, before sending it to a server or attempting to send an email to it, to prevent the most basic mistyping. How could I achieve this?...
5407 votes
75 answers
3.1m views

How do I create a GUID / UUID?

How do I create GUIDs (globally-unique identifiers) in JavaScript? The GUID / UUID should be at least 32 characters and should stay in the ASCII range to avoid trouble when passing them around. I'm ...
5267 votes
106 answers
4.2m views

How do I make the first letter of a string uppercase in JavaScript?

How do I make the first character of a string uppercase if it's a letter, but not change the case of any of the other letters? For example: "this is a test" → "This is a test" &...
Robert Wills's user avatar
  • 53.4k
5195 votes
68 answers
4.9m views

How do I check whether a checkbox is checked in jQuery?

I need to check the checked property of a checkbox and perform an action based on the checked property using jQuery. For example, if the age checkbox is checked, then I need to show a textbox to enter ...
5161 votes
67 answers
3.0m views

What is the most efficient way to deep clone an object in JavaScript?

What is the most efficient way to clone a JavaScript object? I've seen obj = eval(uneval(o)); being used, but that's non-standard and only supported by Firefox. I've done things like obj = JSON.parse(...
4947 votes
62 answers
3.6m views

How do I check if an array includes a value in JavaScript?

What is the most concise and efficient way to find out if a JavaScript array contains a value? This is the only way I know to do it: function contains(a, obj) { for (var i = 0; i < a.length; i++...
brad's user avatar
  • 75k
4857 votes
45 answers
4.4m views

How do I get a timestamp in JavaScript?

I want a single number that represents the current date and time, like a Unix timestamp.
Pablo Fernandez's user avatar
4590 votes
44 answers
3.9m views

Setting "checked" for a checkbox with jQuery

I'd like to do something like this to tick a checkbox using jQuery: $(".myCheckBox").checked(true); or $(".myCheckBox").selected(true); Does such a thing exist?
tpower's user avatar
  • 56.7k
4502 votes
15 answers
855k views

"Thinking in AngularJS" if I have a jQuery background? [closed]

Suppose I'm familiar with developing client-side applications in jQuery, but now I'd like to start using AngularJS. Can you describe the paradigm shift that is necessary? Here are a few questions that ...
Mark Rajcok's user avatar
4490 votes
8 answers
601k views

Why does Google prepend while(1); to their JSON responses?

Why does Google prepend while(1); to their (private) JSON responses? For example, here's a response while turning a calendar on and off in Google Calendar: while (1); [ ['u', [ ['smsSentFlag', ...
Jess's user avatar
  • 42.7k
4435 votes
54 answers
2.5m views

Which "href" value should I use for JavaScript links, "#" or "javascript:void(0)"?

The following are two methods of building a link that has the sole purpose of running JavaScript code. Which is better, in terms of functionality, page load speed, validation purposes, etc.? ...
4343 votes
27 answers
3.0m views

How do I copy to the clipboard in JavaScript?

How do I copy text to the clipboard (multi-browser)? Related: How does Trello access the user's clipboard?
Santiago Corredoira's user avatar
4273 votes
34 answers
3.5m views

How to insert an item into an array at a specific index?

I am looking for a JavaScript array insert method, in the style of: arr.insert(index, item) Preferably in jQuery, but any JavaScript implementation will do at this point.
tags2k's user avatar
  • 83.7k
4218 votes
94 answers
4.6m views

How to round to at most 2 decimal places, if necessary

I'd like to round at most two decimal places, but only if necessary. Input: 10 1.7777777 9.1 Output: 10 1.78 9.1 How can I do this in JavaScript?
stinkycheeseman's user avatar
4169 votes
36 answers
946k views

What is the !! (not not) operator in JavaScript?

I saw this code: this.vertical = vertical !== undefined ? !!vertical : this.vertical; It seems to be using !! as an operator, which I don't recognize. What does it do?
Hexagon Theory's user avatar
4157 votes
61 answers
3.4m views

Sort array of objects by string property value

I have an array of JavaScript objects: var objs = [ { first_nom: 'Laszlo', last_nom: 'Jamf' }, { first_nom: 'Pig', last_nom: 'Bodine' }, { first_nom: 'Pirate', last_nom: '...
Tyrone Slothrop's user avatar
4080 votes
33 answers
3.7m views

Checking if a key exists in a JavaScript object?

How do I check if a particular key exists in a JavaScript object or array? If a key doesn't exist, and I try to access it, will it return false? Or throw an error?
Adam Ernst's user avatar
  • 53.6k
4054 votes
55 answers
5.6m views

How do I check for an empty/undefined/null string in JavaScript?

Is there a string.Empty in JavaScript, or is it just a case of checking for ""?
casademora's user avatar
  • 69.1k
4039 votes
46 answers
5.4m views

Loop through an array in JavaScript

In Java, you can use a for loop to traverse objects in an array as follows: String[] myStringArray = {"Hello", "World"}; for (String s : myStringArray) { // Do something } Can ...
Mark Szymanski's user avatar
3890 votes
41 answers
4.3m views

How do I test for an empty JavaScript object?

After an AJAX request, sometimes my application may return an empty object, like: var a = {}; How can I check whether that's the case?
falmp's user avatar
  • 39.3k
3825 votes
96 answers
5.0m views

What is the JavaScript version of sleep()?

Is there a better way to engineer a sleep in JavaScript than the following pausecomp function (taken from here)? function pausecomp(millis) { var date = new Date(); var curDate = null; do ...
3778 votes
26 answers
3.8m views

Get the current URL with JavaScript?

All I want is to get the website URL. Not the URL as taken from a link. On the page loading I need to be able to grab the full, current URL of the website and set it as a variable to do with as I ...
user avatar
3764 votes
82 answers
2.4m views

How do I correctly clone a JavaScript object?

I have an object x. I'd like to copy it as object y, such that changes to y do not modify x. I realized that copying objects derived from built-in JavaScript objects will result in extra, unwanted ...
soundly_typed's user avatar
3594 votes
48 answers
2.8m views

How do I loop through or enumerate a JavaScript object?

I have a JavaScript object like the following: var p = { "p1": "value1", "p2": "value2", "p3": "value3" }; How do I loop ...
Tanmoy's user avatar
  • 45.3k
3545 votes
108 answers
3.4m views

How can I convert a string to boolean in JavaScript?

Can I convert a string representing a boolean value (e.g., 'true', 'false') into an intrinsic type in JavaScript? I have a hidden form in HTML that is updated based on a user's selection within a list....
3528 votes
71 answers
6.7m views

How do I format a date in JavaScript?

How do I format a Javascript Date object as a string? (Preferably in the format: 10-Aug-2010)
leora's user avatar
  • 194k
3484 votes
25 answers
823k views

What is the difference between call and apply?

What is the difference between using Function.prototype.apply() and Function.prototype.call() to invoke a function? const func = function() { alert("Hello world!"); }; func.apply() vs. ...
John Duff's user avatar
  • 38.4k
3473 votes
43 answers
2.4m views

How to assign a multiline string literal to a variable?

How do I convert this Ruby code with a multiline string into JavaScript? text = <<"HERE" This Is A Multiline String HERE
Newy's user avatar
  • 39.8k
3461 votes
32 answers
1.9m views

pretty-print JSON using JavaScript

How can I display JSON in an easy-to-read (for human readers) format? I'm looking primarily for indentation and whitespace, with perhaps even colors / font-styles / etc.
Mark's user avatar
  • 69k
3428 votes
34 answers
3.5m views

How can I change an element's class with JavaScript?

How can I change the class of an HTML element in response to an onclick or any other events using JavaScript?
Nathan Smith's user avatar
  • 37.4k
3332 votes
51 answers
2.1m views

How can I check if an object is an array? [duplicate]

I'm trying to write a function that either accepts a list of strings, or a single string. If it's a string, then I want to convert it to an array with just the one item so I can loop over it without ...
mpen's user avatar
  • 279k
3324 votes
16 answers
6.4m views

Why does my JavaScript code receive a "No 'Access-Control-Allow-Origin' header is present on the requested resource" error, while Postman does not?

Mod note: This question is about why XMLHttpRequest/fetch/etc. on the browser are subject to the Same Access Policy restrictions (you get errors mentioning CORB or CORS) while Postman is not. This ...
Mr Jedi's user avatar
  • 34.4k
3321 votes
69 answers
2.0m views

How can I merge properties of two JavaScript objects?

I need to be able to merge two (very simple) JavaScript objects at runtime. For example I'd like to: var obj1 = { food: 'pizza', car: 'ford' } var obj2 = { animal: 'dog' } obj1.merge(obj2); //obj1 ...
3252 votes
14 answers
939k views

event.preventDefault() vs. return false

When I want to prevent other event handlers from executing after a certain event is fired, I can use one of two techniques. I'll use jQuery in the examples, but this applies to plain-JS as well: 1. ...
RaYell's user avatar
  • 70.1k
3248 votes
45 answers
484k views

JavaScript closure inside loops – simple practical example

var funcs = []; // let's create 3 functions for (var i = 0; i < 3; i++) { // and store them in funcs funcs[i] = function() { // each should log its value. console.log("My value:", i); ...
nickf's user avatar
  • 543k
3228 votes
49 answers
1.5m views

Detecting an undefined object property

How do I check if an object property in JavaScript is undefined?
Matt Sheppard's user avatar

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