why are initial CSS styles not visible on DOM element.style field?

Asked 23/1, 2019 at 6:54 Answered 23/1, 2019 at 7:4

Solved javascript html css stylesheet getcomputedstyle

OK I have full expectation of going down in flames for asking something stupid (or at least duplicate), but in the attached snippet, why do I have to use window.getComputedStyle to access styles applied by CSS? I was under the impression that the .style field would at least reflect those styles initially applied by CSS, and/or manually changed since then.

If not, what are the exact rules governing which properties are reflected (and when) in an element's .style field?

setTimeout(() => {
    console.log("the bckg color:", reddish.style.backgroundColor);
    console.log("the width:", reddish.style.width);
    console.log("from a computed style:", window.getComputedStyle(reddish).backgroundColor);
    console.log("from a computed style:", window.getComputedStyle(reddish).width);
}, 100);

#reddish {
    background-color: #fa5;
    width: 100px;
    height: 100px;
}

<html>
    <body>
        <div id="reddish"></div>
    </body>
</html>

Emmetropia answered 23/1, 2019 at 6:54 Comment(0)

The HTMLElement.style property is not useful for completely learning about the styles applied on the element, since it represents only the CSS declarations set in the element's inline style attribute, not those that come from style rules elsewhere, such as style rules in the section, or external style sheets. To get the values of all CSS properties for an element you should use Window.getComputedStyle() instead.

Via- MDN Web Docs | Getting Style Information

HTMLElement.style:

The HTMLElement.style property is used to get as well as set the inline style of an element.

console.log(document.getElementById("para").style.fontSize); // will work since the "font-size" property is set inline
console.log(document.getElementById("para").style.color); // will not work since the "color" property is not set inline

#para {color: rgb(34, 34, 34);}

<p id="para" style="font-size: 20px;">Hello</p>

Window.getComputedStyle():

The getComputedStyle() method however, returns an object containing the values of all CSS properties of an element, after applying active stylesheets and resolving any basic computation those values may contain thus returning the css properties from both inline style declarations as well as from external style-sheets.

console.log(window.getComputedStyle(document.getElementById("para")).fontSize); // will work
console.log(window.getComputedStyle(document.getElementById("para")).color); // will work

#para {
  color: rgb(34, 34, 34);
}

<p id="para" style="font-size: 20px;">Hello</p>

Mansell answered 23/1, 2019 at 7:4 Comment(2)

You should probably have quoted this paragraph instead. Ps: quote blocks are supported in the editor. – Tussle 23/1, 2019 at 7:26

@Tussle Indeed. Let me update the answer. Thanks for the heads up man. – Mansell 23/1, 2019 at 7:28

HTMLElement.style is for the inline style of an element. It does not take into account CSS whatsoever. This is basically just directly setting or getting a property on the element object.

<div style="color: red;">Hello</div>

Window.getComputedStyle() takes into account inline styles and CSS, after resolving cascading, inheritance, etc. It's basically the "final" actual style value used to render the element on the page.

// CSS
#blue-text {
  color: blue !important;
}

// HTML
<div style="color: red;" id="blue-text">Hello</div>

// JS
const myElement = document.querySelector("#blue-text");
myElement.style.color; // "red" because that's the inline style
window.getComputedStyle(myElement).color; // "rgb(0, 0, 255)" because CSS !important overrides inline style

Shaniqua answered 23/1, 2019 at 7:0 Comment(4)

OK, so basically, I can trust HTMLElement.style if and only if either either: (a) I am accessing a style property that was defined inline (and not overridden by some !important CSS declaration), or (b) I am accessing a style property that was previously set via element.style.property = ...? – Emmetropia 23/1, 2019 at 7:8

Basically yeah. – Shaniqua 23/1, 2019 at 7:9

@Emmetropia that depends what you expect it to mean. You can't ever trust Element.style to return the applied value of a CSS property. It can very well be overridden by an !important rule somewhere else, or even just be some value with no real meaning like "auto". – Tussle 23/1, 2019 at 7:19

...except that using element.style.property = ... still does not override an !important CSS style, apparently. So the rule is really: using HMTLElement.style is like editing the inline style attribute of the element, nothing more. (@Tussle our comments crossed. Thanks.) – Emmetropia 23/1, 2019 at 7:20

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