When to use setAttribute vs .attribute= in JavaScript?

These answers aren't really addressing the large confusion with between properties and attributes. Also, depending on the Javascript prototype, sometimes you can use a an element's property to access an attributes and sometimes you can't.

First, you have to remember that an HTMLElement is a Javascript object. Like all objects, they have properties. Sure, you can create a property called nearly anything you want inside HTMLElement, but it doesn't have to do anything with the DOM (what's on the page). The dot notation (.) is for properties. Now, there some special properties that are mapped to attributes, and at the time or writing there are only 4 that are guaranteed (more on that later).

All HTMLElements include a property called attributes. HTMLElement.attributes is a live NamedNodeMap Object that relates to the elements in the DOM. "Live" means that when the node changes in the DOM, they change on the JavaScript side, and vice versa. DOM attributes, in this case, are the nodes in question. A Node has a .nodeValue property that you can change. NamedNodeMap objects have a function called setNamedItem where you can change the entire node. You can also directly access the node by the key. For example, you can say .attributes["dir"] which is the same as .attributes.getNamedItem('dir'); (Side note, NamedNodeMap is case-insensitive, so you can also pass 'DIR');

There's a similar function directly in HTMLElement where you can just call setAttribute which will automatically create a node if it doesn't exist and set the nodeValue. There are also some attributes you can access directly as properties in HTMLElement via special properties, such as dir. Here's a rough mapping of what it looks like:

HTMLElement {
  attributes: {
    setNamedItem: function(attr, newAttr) { 
      this[attr] = newAttr;
    },    
    getNamedItem: function(attr) {
      return this[attr];
    },
    myAttribute1: {
      nodeName: 'myAttribute1',
      nodeValue: 'myNodeValue1'
    },
    myAttribute2: {
      nodeName: 'myAttribute2',
      nodeValue: 'myNodeValue2'
    },
  }
  setAttribute: function(attr, value) { 
    let item = this.attributes.getNamedItem(attr);
    if (!item) {
      item = document.createAttribute(attr);
      this.attributes.setNamedItem(attr, item);
    }
    item.nodeValue = value;
  },
  getAttribute: function(attr) { 
    return this.attributes[attr] && this.attributes[attr].nodeValue;
  },
  dir: // Special map to attributes.dir.nodeValue || ''
  id:  // Special map to attributes.id.nodeValue || ''
  className: // Special map to attributes.class.nodeValue || '' 
  lang: // Special map to attributes.lang.nodeValue || ''

}

So you can change the dir attributes 6 ways:

  // 1. Replace the node with setNamedItem
  const newAttribute = document.createAttribute('dir');
  newAttribute.nodeValue = 'rtl';
  element.attributes.setNamedItem(newAttribute);

  // 2. Replace the node by property name;
  const newAttribute2 = document.createAttribute('dir');
  newAttribute2.nodeValue = 'rtl';
  element.attributes['dir'] = newAttribute2;
  // OR
  element.attributes.dir = newAttribute2;

  // 3. Access node with getNamedItem and update nodeValue
  // Attribute must already exist!!!
  element.attributes.getNamedItem('dir').nodeValue = 'rtl';

  // 4. Access node by property update nodeValue
  // Attribute must already exist!!!
  element.attributes['dir'].nodeValue = 'rtl';
  // OR
  element.attributes.dir.nodeValue = 'rtl';

  // 5. use setAttribute()  
  element.setAttribute('dir', 'rtl');
  
  // 6. use the UNIQUELY SPECIAL dir property
  element["dir"] = 'rtl';
  element.dir = 'rtl';

You can update all properties with methods #1-5, but only dir, id, lang, and className with method #6.

Extensions of HTMLElement

HTMLElement has those 4 special properties. Some elements are extended classes of HTMLElement have even more mapped properties. For example, HTMLAnchorElement has HTMLAnchorElement.href, HTMLAnchorElement.rel, and HTMLAnchorElement.target. But, beware, if you set those properties on elements that do not have those special properties (like on a HTMLTableElement) then the attributes aren't changed and they are just, normal custom properties. To better understand, here's an example of its inheritance:

HTMLAnchorElement extends HTMLElement {
  // inherits all of HTMLElement
  href:    // Special map to attributes.href.nodeValue || ''
  target:  // Special map to attributes.target.nodeValue || ''
  rel:     // Special map to attributes.ref.nodeValue || '' 
}

Custom Properties

Now the big warning: Like all Javascript objects, you can add custom properties. But, those won't change anything on the DOM. You can do:

  const newElement = document.createElement('div');
  // THIS WILL NOT CHANGE THE ATTRIBUTE
  newElement.display = 'block';

But that's the same as

  newElement.myCustomDisplayAttribute = 'block';

This means that adding a custom property will not be linked to .attributes[attr].nodeValue.

Performance

I've built a jsperf test case to show the difference: https://jsperf.com/set-attribute-comparison. Basically, In order:

1. Custom properties because they don't affect the DOM and are not attributes.
2. Special mappings provided by the browser (dir, id, className).
3. If attributes already exists, element.attributes.ATTRIBUTENAME.nodeValue =
4. setAttribute();
5. If attributes already exists, element.attributes.getNamedItem(ATTRIBUTENAME).nodeValue = newValue
6. element.attributes.ATTRIBUTENAME = newNode
7. element.attributes.setNamedItem(ATTRIBUTENAME) = newNode

Conclusion (TL;DR)

• Use the special property mappings from HTMLElement: element.dir, element.id, element.className, or element.lang.

• If you are 100% sure the element is an extended HTMLElement with a special property, use that special mapping. (You can check with if (element instanceof HTMLAnchorElement)).

• If you are 100% sure the attribute already exists, use element.attributes.ATTRIBUTENAME.nodeValue = newValue.

• If not, use setAttribute().

Interesting takeout from Google API script regarding this:

They do it like this:

var scriptElement = document.createElement("script");
scriptElement = setAttribute("src", "https://some.com");
scriptElement = setAttribute("nonce", "https://some.com");
scriptElement.async = "true";

Notice, how they use setAttribute for "src" and "nonce", but then .async = ... for "async" attribute.

I'm not 100% sure, but probably that's because "async" is only supported on browsers that support direct .attr = assignment. So, there's no sense trying to sestAttribute("async") because if browser doesn't understand .async=... - it will not understand "async" attribute.

Hopefully, that's a helpful insight from my ongoing "Un-minify GAPI" research project. Correct me if I'm wrong.

This is very good discussion. I had one of those moments when I wished or lets say hoped (successfully that I might add) to reinvent the wheel be it a square one. Any ways above is good discussion, so any one coming here looking for what is the difference between Element property and attribute. here is my penny worth and I did have to find it out hard way. I would keep it simple so no extraordinary tech jargon.

suppose we have a variable calls 'A'. what we are used to is as following.

Below will throw an error because simply it put its is kind of object that can only have one property and that is singular left hand side = singular right hand side object. Every thing else is ignored and tossed out in bin.

let A = 'f';
A.b =2;
console.log(A.b);

who has decided that it has to be singular = singular. People who make JavaScript and html standards and thats how engines work.

Lets change the example.

let A = {};
A.b =2;
console.log(A.b);

This time it works ..... because we have explicitly told it so and who decided we can tell it in this case but not in previous case. Again people who make JavaScript and html standards.

I hope we are on this lets complicate it further

let A = {};
A.attribute ={};
A.attribute.b=5;
console.log(A.attribute.b); // will work

console.log(A.b); // will not work

What we have done is tree of sorts level 1 then sub levels of non-singular object. Unless you know what is where and and call it so it will work else no.

This is what goes on with HTMLDOM when its parsed and painted a DOm tree is created for each and every HTML ELEMENT. Each has level of properties per say. Some are predefined and some are not. This is where ID and VALUE bits come on. Behind the scene they are mapped on 1:1 between level 1 property and sun level property aka attributes. Thus changing one changes the other. This is were object getter ans setter scheme of things plays role.

let A = {
attribute :{
id:'',
value:''
},
getAttributes: function (n) {
    return this.attribute[n];
  },
setAttributes: function (n,nn){
this.attribute[n] = nn;
if(this[n]) this[n] = nn;
},
id:'',
value:''
};



A.id = 5;
console.log(A.id);
console.log(A.getAttributes('id'));

A.setAttributes('id',7)
console.log(A.id);
console.log(A.getAttributes('id'));

A.setAttributes('ids',7)
console.log(A.ids);
console.log(A.getAttributes('ids'));

A.idsss=7;
console.log(A.idsss);
console.log(A.getAttributes('idsss'));

This is the point as shown above ELEMENTS has another set of so called property list attributes and it has its own main properties. there some predefined properties between the two and are mapped as 1:1 e.g. ID is common to every one but value is not nor is src. when the parser reaches that point it simply pulls up dictionary as to what to when such and such are present.

All elements have properties and attributes and some of the items between them are common. What is common in one is not common in another.

In old days of HTML3 and what not we worked with html first then on to JS. Now days its other way around and so has using inline onlclick become such an abomination. Things have moved forward in HTML5 where there are many property lists accessible as collection e.g. class, style. In old days color was a property now that is moved to css for handling is no longer valid attribute.

Element.attributes is sub property list with in Element property.

Unless you could change the getter and setter of Element property which is almost high unlikely as it would break hell on all functionality is usually not writable off the bat just because we defined something as A.item does not necessarily mean Js engine will also run another line of function to add it into Element.attributes.item.

I hope this gives some headway clarification as to what is what. Just for the sake of this I tried Element.prototype.setAttribute with custom function it just broke loose whole thing all together, as it overrode native bunch of functions that set attribute function was playing behind the scene.

Adding 2 more points related to .textContent and .innerHTML

<div id="mydiv"></div>


var elem = document.getElementById("mydiv");

elem.textContent = "hello"; // OK - Content has been updated
elem.setAttribute("textContent", "hello"); // NOT OK - You are trying to set
                                           // the attribute than it's content

elem.innerHTML = "world";   // OK - Content has been updated
elem.setAttribute("innerHTML", "world"); // NOT OK - You are trying to set
                                         // the attribute than it's content

One difference between the two is that setAttribute, when used to set the value of an <input/> will make that the default value when you call .reset() on the form it's part of, but .value = will not do this.

https://developer.mozilla.org/en-US/docs/Web/API/HTMLFormElement/reset

Note that if setAttribute() is called to set the value of a particular attribute, a subsequent call to reset() won’t reset the attribute to its default value, but instead will keep the attribute at whatever value the setAttribute() call set it to.

If the element you are referring to does not already include a Javascript object property for a given attribute (as others have described), then setting that property will not propagate the change back to the DOM, it just adds the named property to the Javascript object, and the DOM ignores it. For example, getting the element mySpan by id and then doing mySpan.class = 'warning' will do nothing, whether or not the span element in question already has a class attribute defined, because mySpan.class is not defined in the Javascript object for a span element. You have to use mySpan.setAttribute('class', 'warning').

However a second nuance is that setting the Javascript object's innerHTML property using mySpan.setAttribute("innerHTML", someHTML) does not update the content of the element. I don't know how Javascript even catches mySpan.innerHTML = something, and calls the HTML parser, but there's some magic going on under the hood.