There are generally two ways to style an element:
- Create a class in CSS and add it:
- Write properties directly into
We should always prefer CSS classes to
style. The latter should only be used if classes “can’t handle it”.
let top = /* complex calculations */; let left = /* complex calculations */; elem.style.left = left; // e.g '123px', calculated at run-time elem.style.top = top; // e.g '456px'
className and classList
Changing a class is one of the most often used actions in scripts.
"class" could not be an object property. That limitation does not exist now, but at that time it was impossible to have a
"class" property, like
So for classes the similar-looking property
"className" was introduced: the
elem.className corresponds to the
If we assign something to
elem.className, it replaces the whole string of classes. Sometimes that’s what we need, but often we want to add/remove a single class.
There’s another property for that:
elem.classList is a special object with methods to
add/remove/toggle a single class.
So we can operate both on the full class string using
className or on individual classes using
classList. What we choose depends on our needs.
elem.classList.add/remove("class")– adds/removes the class.
elem.classList.toggle("class")– adds the class if it doesn’t exist, otherwise removes it.
elem.classList.contains("class")– checks for the given class, returns
classList is iterable, so we can list all classes with
for..of, like this:
elem.style is an object that corresponds to what’s written in the
"style" attribute. Setting
elem.style.width="100px" works the same as if we had in the attribute
style a string
For multi-word property the camelCase is used:
background-color => elem.style.backgroundColor z-index => elem.style.zIndex border-left-width => elem.style.borderLeftWidth
Browser-prefixed properties like
-webkit-border-radius also follow the same rule: a dash means upper case.
button.style.MozBorderRadius = '5px'; button.style.WebkitBorderRadius = '5px';
Resetting the style property
Sometimes we want to assign a style property, and later remove it.
For instance, to hide an element, we can set
elem.style.display = "none".
Then later we may want to remove the
style.display as if it were not set. Instead of
delete elem.style.display we should assign an empty string to it:
elem.style.display = "".
If we set
style.display to an empty string, then the browser applies CSS classes and its built-in styles normally, as if there were no such
style.display property at all.
Also there is a special method for that,
elem.style.removeProperty('style property'). So, We can remove a property like this:
Normally, we use
style.* to assign individual style properties. We can’t set the full style like
div.style="color: red; width: 100px", because
div.style is an object, and it’s read-only.
To set the full style as a string, there’s a special property
This property is rarely used, because such assignment removes all existing styles: it does not add, but replaces them. May occasionally delete something needed. But we can safely use it for new elements, when we know we won’t delete an existing style.
The same can be accomplished by setting an attribute:
div.setAttribute('style', 'color: red...').
Mind the units
Don’t forget to add CSS units to values.
For instance, we should not set
10, but rather to
10px. Otherwise it wouldn’t work:
<body> <script> // doesn't work! document.body.style.margin = 20; alert(document.body.style.margin); // '' (empty string, the assignment is ignored) // now add the CSS unit (px) - and it works document.body.style.margin = '20px'; alert(document.body.style.margin); // 20px alert(document.body.style.marginTop); // 20px alert(document.body.style.marginLeft); // 20px </script> </body>
Please note: the browser “unpacks” the property
style.margin in the last lines and infers
style.marginTop from it.
Computed styles: getComputedStyle
So, modifying a style is easy. But how to read it?
For instance, we want to know the size, margins, the color of an element. How to do it?
style property operates only on the value of the
"style" attribute, without any CSS cascade.
So we can’t read anything that comes from CSS classes using
For instance, here
style doesn’t see the margin:
…But what if we need, say, to increase the margin by
20px? We would want the current value of it.
There’s another method for that:
The syntax is:
- Element to read the value for.
- A pseudo-element if required, for instance
::before. An empty string or no argument means the element itself.
The result is an object with styles, like
elem.style, but now with respect to all CSS classes.
There are two concepts in CSS:
- A computed style value is the value after all CSS rules and CSS inheritance is applied, as the result of the CSS cascade. It can look like
- A resolved style value is the one finally applied to the element. Values like
125%are relative. The browser takes the computed value and makes all units fixed and absolute, for instance:
font-size:16px. For geometry properties resolved values may have a floating point, like
A long time ago
getComputedStyle was created to get computed values, but it turned out that resolved values are much more convenient, and the standard changed.
getComputedStyle actually returns the resolved value of the property, usually in
px for geometry.
getComputedStylerequires the full property name
We should always ask for the exact property that we want, like
borderTopWidth. Otherwise the correct result is not guaranteed.
For instance, if there are properties
paddingLeft/paddingTop, then what should we get for
getComputedStyle(elem).padding? Nothing, or maybe a “generated” value from known paddings? There’s no standard rule here.
:visitedlinks are hidden!
Visited links may be colored using
:visited CSS pseudoclass.
getComputedStyle does not give access to that color, because otherwise an arbitrary page could find out whether the user visited a link by creating it on the page and checking the styles.
:visited. And also, there’s a limitation in CSS that forbids applying geometry-changing styles in
:visited. That’s to guarantee that there’s no side way for an evil page to test if a link was visited and hence to break the privacy.
To manage classes, there are two DOM properties:
className– the string value, good to manage the whole set of classes.
classList– the object with methods
add/remove/toggle/contains, good for individual classes.
To change the styles:
styleproperty is an object with camelCased styles. Reading and writing to it has the same meaning as modifying individual properties in the
"style"attribute. To see how to apply
importantand other rare stuff – there’s a list of methods at MDN.
style.cssTextproperty corresponds to the whole
"style"attribute, the full string of styles.
To read the resolved styles (with respect to all classes, after all CSS is applied and final values are calculated):
getComputedStyle(elem, [pseudo])returns the style-like object with them. Read-only.
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