Why is using onClick in HTML a bad practice

Question

I have heard many times that using JavaScript events  such as onClick    in HTML is a bad practice  because it s not good for semantics  I would like to know what the downsides are and how to fix the following code    lt a href     onclick  popup   map    300  300   map    return false   gt link lt  a gt

User · Answer

onclick events run in the global scope and may cause unexpected error   Adding onclick events to many DOM elements will slow down the performance and efficiency

User · Answer

There are a few reasons    I find it aids maintenence to separate markup  i e  the HTML and client-side scripts  For example  jQuery makes it easy to add event handlers programatically  The example you give would be broken in any user agent that doesn t support javascript  or has javascript turned off  The concept of progressive enhancement would encourage a simple hyperlink to  map  for user agents without javascript  then adding a click handler prgramatically for user agents that support javascript    For example   Markup    lt a id  example  href   map   gt link lt  a gt    Javascript     document  ready function             example   click function            popup   map    300  300   map            return false

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Two more reasons not to use inline handlers  They can require tedious quote escaping issues Given an arbitrary string  if you want to be able to construct an inline handler that calls a function with that string  for the general solution  you ll have to escape the attribute delimiters  with the associated HTML entity   and you ll have to escape the delimiter used for the string inside the attribute  like the following   x000D   x000D  const str   prompt  What string to display on click     foo   bar    const escapedStr   str      since the attribute value is going to be using   delimiters       replace  s with their corresponding HTML entity     replace    g    amp quot         since the string literal inside the attribute is going to delimited with  s       escape  s     replace    g             document body insertAdjacentHTML     beforeend       lt button onclick  alert       escapedStr         gt click lt  button gt      x000D   x000D   x000D   That s incredibly ugly  From the above example  if you didn t replace the  s  a SyntaxError would result  because alert  foo  quot bar   is not valid syntax  If you didn t replace the  quot s  then the browser would interpret it as an end to the onclick attribute  delimited with  quot s above   which would also be incorrect  If one habitually uses inline handlers  one would have to make sure to remember do something similar to the above  and do it right  every time  which is tedious and hard to understand at a glance  Better to avoid inline handlers entirely so that the arbitrary string can be used in a simple closure   x000D   x000D  const str   prompt  What string to display on click     foo   bar    const button   document body appendChild document createElement  button     button textContent    click   button onclick        gt  alert str   x000D   x000D   x000D   Isn t that so much nicer   The scope chain of an inline handler is extremely peculiar What do you think the following code will log   x000D   x000D  let disabled   true  x000D   lt form gt     lt button onclick  console log disabled    gt click lt  button gt   lt  form gt  x000D   x000D   x000D   Try it  run the snippet  It s probably not what you were expecting  Why does it produce what it does  Because inline handlers run inside with blocks  The above code is inside three with blocks  one for the document  one for the  lt form gt   and one for the  lt button gt    x000D   x000D  let disabled   true  x000D   lt form gt     lt button onclick  console log disabled    gt click lt  button gt   lt  form gt  x000D   x000D   x000D    Since disabled is a property of the button  referencing disabled inside the inline handler refers to the button s property  not the outer disabled variable  This is quite counter-intuitive  with has many problems  it can be the source of confusing bugs and significantly slows down code  It isn t even permitted at all in strict mode  But with inline handlers  you re forced to run the code through withs - and not just through one with  but through multiple nested withs  It s crazy  with should never be used in code  Because inline handlers implicitly require with along with all its confusing behavior  inline handlers should be avoided as well

User · Answer

It s a new paradigm called  Unobtrusive JavaScript    The current  web standard  says to separate functionality and presentation   It s not really a  bad practice   it s just that most new standards want you to use event listeners instead of in-lining JavaScript   Also  this may just be a personal thing  but I think it s much easier to read when you use event listeners  especially if you have more than 1 JavaScript statement you want to run

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Your question will trigger discussion I suppose  The general idea is that it s good to separate behavior and structure  Furthermore  afaik  an inline click handler has to be evalled to  become  a real javascript function  And it s pretty old fashioned  allbeit that that s a pretty shaky argument  Ah  well  read some about it  quirksmode org

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It s not good for several reasons    it mixes code and markup code written this way goes through eval and runs in the global scope   The simplest thing would be to add a name attribute to your  lt a gt  element  then you could do   document myelement onclick   function         window popup   map    300  300   map        return false       although modern best practise would be to use an id instead of a name  and use addEventListener   instead of using onclick since that allows you to bind multiple functions to a single event

User · Answer

You re probably talking about unobtrusive Javascript  which would look like this    lt a href     id  someLink  gt link lt  a gt    with the logic in a central javascript file looking something like this       someLink   click function        popup   map    300  300   map         return false        The advantages are   behaviour  Javascript  is separated from presentation  HTML  no mixing of languages you re using a javascript framework like jQuery that can handle most cross-browser issues for you You can add behaviour to a lot of HTML elements at once without code duplication

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If you are using jQuery then   HTML     lt a id  openMap  href   map   gt link lt  a gt    JS     document  ready function             openMap   click function            popup   map    300  300   map            return false                This has the benefit of still working without JS  or if the user middle clicks the link   It also means that I could handle generic popups by rewriting again to   HTML     lt a class  popup  href   map   gt link lt  a gt    JS     document  ready function             popup   click function            popup   this  attr  href    300  300   map            return false                This would let you add a popup to any link by just giving it the popup class   This idea could be extended even further like so   HTML     lt a class  popup  data-width  300  data-height  300  href   map   gt link lt  a gt    JS     document  ready function             popup   click function            popup   this  attr  href      this  data  width      this  data  height     map            return false                I can now use the same bit of code for lots of popups on my whole site without having to write loads of onclick stuff  Yay for reusability   It also means that if later on I decide that popups are bad practice   which they are   and that I want to replace them with a lightbox style modal window  I can change   popup   this  attr  href      this  data  width      this  data  height     map      to  myAmazingModalWindow   this  attr  href      this  data  width      this  data  height     map      and all my popups on my whole site are now working totally differently  I could even do feature detection to decide what to do on a popup  or store a users preference to allow them or not  With the inline onclick  this requires a huge copy and pasting effort

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With very large JavaScript applications  programmers are using more encapsulation of code to avoid polluting the global scope  And to make a function available to the onClick action in an HTML element  it has to be in the global scope   You may have seen JS files that look like this      function            some code          These are Immediately Invoked Function Expressions  IIFEs  and any function declared within them will only exist within their internal scope    If you declare function doSomething     within an IIFE  then make doSomething   an element s onClick action in your HTML page  you ll get an error   If  on the other hand  you create an eventListener for that element within that IIFE and call doSomething   when the listener detects a click event  you re good because the listener and doSomething   share the IIFE s scope   For little web apps with a minimal amount of code  it doesn t matter  But if you aspire to write large  maintainable codebases  onclick    is a habit that you should work to avoid

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Revision  Unobtrusive JavaScript approach was good in the PAST - especially events handler bind in HTML was considered as bad practice  mainly because onclick events run in the global scope and may cause unexpected error what was mention by YiddishNinja   However     Currently it seems that this approach is a little outdated and needs some update  If someone want to be professional frontend developper and write large and complicated apps then he need to use frameworks like Angular  Vue js  etc    However that frameworks usually use  or allow to use  HTML-templates where event handlers are bind in html-template code directly and this is very handy  clear and effective - e g  in angular template usually people write    lt button  click   someAction    gt Click Me lt  button gt     In raw js html the equivalent of this will be   lt button onclick  someAction    gt Click Me lt  button gt    The difference is that in raw js onclick event is run in the global scope - but the frameworks provide encapsulation   So where is the problem   The problem is when novice programmer who always heard that html-onclick is bad and who always use btn addEventListener  onclick         wants to use some framework with templates  addEventListener also have drawbacks - if we update DOM in dynamic way using innerHTML   which is pretty fast  - then we loose events handlers bind in that way   Then he will face something like bad-habits or wrong-approach to framework usage - and he will use framework in very bad way - because he will focus mainly on js-part and no on template-part  and produce unclear and hard to maintain code   To change this habits he will loose a lot of time  and probably he will need some luck and teacher    So in my opinion  based on experience with my students  better would be for them if they use html-handlers-bind at the beginning  As I say it is true that handlers are call in global scope but a this stage students usually create small applications which are easy to control  To write bigger applications they choose some frameworks   So what to do   We can UPDATE the Unobtrusive JavaScript approach and allow bind event handlers  eventually with simple parameters  in html  but only bind handler - not put logic into onclick like in OP quesiton   So in my opinion in raw js html this should be allowed   lt button onclick  someAction 3   gt Click Me lt  button gt    or   x000D   x000D  function popup num str event    x000D     let re new RegExp str     x000D             x000D     event preventDefault    x000D     console log  link was clicked    x000D    x000D   lt a href  https   example com  onclick  popup 300  map  event   gt link lt  a gt  x000D   x000D   x000D    But below examples should NOT be allowed   lt button onclick  console log  xx    someAction    return true  gt Click Me lt  button gt    lt a href     onclick  popup   map    300  300   map    return false   gt link lt  a gt    The reality changes  our point of view should too

[javascript] Why is using onClick() in HTML a bad practice?

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