How many times has it happened with you when you specify a wrong thing into a CSS variable and spend hours debugging? Check out the new at-rule called @property
.
What is @property
?
The @property
is part of CSS Houdini. It allows developers to define custom css properties. You might think, "Isn't that the point of CSS variables?". Well, when we use the var()
function in css, it reads the variable as a string. When we use @property
, it actually reads the data. It's basically css variables with superpowers.
Before @property
came into the picture, we had to add javascript code to register a custom property.
CSS.registerProperty({
name: '--color',
syntax: '<color>',
initialValue: 'yellow',
inherits: false
});
With the new @property
, we can do something like this -
@property --color {
syntax: "<color>";
initial-value: magenta;
inherits: false;
}
Here we are defining a property called --color
. We have specified a syntax
field which says the type of value the variable should have. The some of the possible values of syntax
are -
- length
- number
- color
- percentage
- url
- integer
- angle
- resolution
NOTE:- There may be more properties in the future, there is not a lot of documentation about this yet but I expect that this should improve.
Let's look at some code to understand this better.
@property --color {
syntax: "<color>";
initial-value: yellow;
inherits: false;
}
We have made a property called --color
. We have specified that the syntax to check for is a color. This includes hex codes, rgb etc. We've set the initial-value
to yellow
. I'll explain what exactly initial-value
does later. For now, let's add some code that uses this property.
.box {
background: var(--color); /* yellow */
}
The output is yellow
. This is where the initial-value
comes into play. Since we have not specified a --color
variable inside .box
, the color in the initial-value
will be used.
Let's make a small change and define a variable of the same name as the property.
.box {
--color: cyan;
background: var(--color); /* cyan */
}
Now, our output looks like this -
The color of the box is cyan
as the --color
variable overrides the intial-value
of the --color
property. You might say, "Akash, why can't I just use a CSS variable?". Remember the syntax
property we specified? Let's understand what it means by making a tiny change in our code -
.box {
--color: 234; /* Set the value to a number */
background: var(--color); /* cyan */
}
Look at the output:
As you can see, the color is yellow. This is because the value we set for the --color
variable does not satisfy the <color>
value we set for the syntax
field. So, the property uses the initial-value
i.e. yellow
.
Check out the overall demo at this codepen:
Important Note on Browser Support
One thing you guys should keep in mind is that @property
is experimental. It's available on Chrome, Opera and a few other browsers. I would not recommend using this in production level apps without fallbacks as of now.Check this link to know more
Why use @property
?
Fallbacks
Let's imagine a developer forgot to define the property on the body and just usedvar(--whateverName)
. The value will fallback to theinitial-value
on the@property
.Validation
More often than not, the variable may have a different unit than the one required. For example, a developer may specify a number instead of a percentage. In these cases, the value specified may not be same as the value of thesyntax
property. The value will fall back to theinitial-value
and prevent bugs.
Thank you for reading until this point, that's all for now. Hope you guys liked this post and learned something new. If so, like the post and follow me. Have a good day, bye 👋
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