SCSS rem fallback mixin, my take

The rem CSS unit allows you to base your font sizes off of the user’s configured font size, but not be affected by parent elements like ems are. Older browsers don’t support rems though, so it’s good to provide a fallback in px for them by defining the property with a px value, then a rem value. Old browsers take the px value, then see they don’t know how to handle the rem value and ignore it. New browsers take the px value, then override it with the rem value.

There are a number of SCSS mixins out there for making rem fallbacks automatically by passing in a property and some values and having it automatically output both versions of the property. I started with the css-tricks one. None of the ones I found, though, worked exactly as I wanted. I wanted to be able to work with any property values. I wanted:

  • unitless number values (other than 0) to be treated as px values
  • px or rem values to be converted to the other unit
  • values with other units or non-numbers to be output as is (auto, none, url, etc)

This would allow things like:

a{
    @include remFallback(background, url('foo.jpeg') 1em 24);
    @include remFallback(margin, 0 auto 10px 1em);
}

CSS output:

a{
    background: url('foo.jpeg') 1em 24px;
    background: url('foo.jpeg') 1em 2.4rem;
    margin: 0 auto 10px 1em;
    margin: 0 auto 1rem 1em;
}

making it much more versatile. Hugo Giraudel made a nice version closer to what I wanted. I took that and modified it to be closer still. It seems to work for any case except for values with commas (such as multiple backgrounds). It can be found in my html boilerplate. Currently, it looks like:

/*
Variable: fontSizes.root
'px' font size on root (html) element (with units removed).  Browser default is 16px.  Used to determine sizes for remFallback function.

-@ based on http://css-tricks.com/snippets/css/less-mixin-for-rem-font-sizing/
*/
$fontSizes: () !default;
$fontSizes: map-merge((root: 16), $fontSizes);
/*
Mixin: remFallback
Output a property with pixel values for non 'rem' supporting browsers followed by the 'rem' equivalent for rem support browsers.  Can't currently accept values with comma separation, like multiple backgrounds or the like.

-@ based on http://hugogiraudel.com/2013/03/18/ultimate-rem-mixin/
*/

@mixin remFallback($property, $values) {
    $px: ();
    $rem: ();

    @each $value in $values{
        //--get unit or 'nan' if the value isn't a number
        // $unit: if(type-of($value) == 'number', unit($value), 'nan');
        $unit: 'nan';
        @if type-of($value) == 'number'{
            @if unitless($value){
                $unit: 'unitless';
            }@else{
                $unit: unit($value);
            }
        }

        //--unitless numbers are added as if they are in pix
        @if $unit == 'unitless' and $value != 0{
            $px : append($px, $value + px);
            $rem: append($rem, ($value / map-get($fontSizes, root) + rem));
        //--px or rem get added to their list and then converted to the other unit
        }@else if $unit == 'px' or $unit == 'rem'{
            $unitlessVal: stripUnit($value);
            @if $unit == 'px'{
                $px : append($px, $value);
                $rem: append($rem, ($unitlessVal / map-get($fontSizes, root) + rem));
            }@else if $unit == 'rem'{
                $px : append($px,($unitlessVal * map-get($fontSizes, root) + px));
                $rem: append($rem, $value);
            }
        //--all other units get appended directly
        }@else{
            $px : append($px , $value);
            $rem: append($rem, $value);
        }
    }

    @if $px == $rem {
        #{$property}: $px;
    } @else {
        #{$property}: $px;
        #{$property}: $rem;
    }
}

/*
Mixin: stripUnit
Strip the unit from a value that has a unit

-@ based on http://css-tricks.com/snippets/css/less-mixin-for-rem-font-sizing/
*/
@function stripUnit($num) {
    @return $num / ($num * 0 + 1);
}

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Toby

I am a quiet person from Northeast Ohio. I work as a web developer. I like computers, music, and many other things.

4 thoughts on “SCSS rem fallback mixin, my take”

    1. The code in this post got messed up during a site migration apparently. I’ve updated it and fixed the link to the code on github. Hopefully that fixes the problem.

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