# Responsive font size mixin [![npm][npm-image]][npm-url]
[npm-image]: https://img.shields.io/npm/v/rfs.svg
[npm-url]: https://npmjs.org/package/rfs
> This package lets you use the `responsive-font-size` mixin which **automatically calculates the appropriate font 
size** based on the dimensions of the monitor or device.

## What is RFS?
RFS (abbreviation for responsive font size) is the name of the algorithm behind the mixin. It's also used by 
[PostCSS RFS](https://github.com/MartijnCuppens/postcss-rfs) and [Less RFS](https://github.com/MartijnCuppens/less-rfs).

## Advantages
- Font sizes will **rescale for every screen or device**, this prevents long words from being chopped off the screen on 
mobile devices.
- The minimum font size (configuration variable) will prevent the font size from becoming too small so readability can 
be assured.
- **Super easy** to use, no need to define complex configurations for each font size.
- Font sizes of all text elements will always remain in relation with each other.


![RFS](http://i.imgur.com/gJH6m6g.gif)

## Installation
You can use RFS in your project by installing it using a package manager (recommended):

**npm:**

```
$ npm install rfs --save
```

**yarn:**

```
$ yarn add rfs
```

**Bower:**

```
$ bower install rfs --save
```

**Copy/paste (not recommended):**

You can download the RFS SCSS-file and save it in your `scss/` directory. This method is not recommended because you 
lose the ability to easily and quickly manage and update RFS as a dependency.


## Usage
This input (SCSS):
```scss
.title {
  @include responsive-font-size(4rem);
  // OR:
  // @include responsive-font-size(64px);
  // OR:
  // @include responsive-font-size(64);
}
```

Will generate this (CSS):
```css
.title,
.disable-responsive-font-size .title,
.title.disable-responsive-font-size  {
  font-size: 4rem;
}

@media (max-width: 1200px) {
  .title {
    font-size: calc(1.6rem + 3.2vw);
  }
}
```
You can also use the `rfs()` alias instead of `responsive-font-size()`.


## Configuration

![RFS visualisation](https://i.imgur.com/9YciUbb.png)

**$rfs-minimum-font-size:** (in `px` or `rem`)  
Font sizes which are calculated by RFS will never be lower than this size. However, you can still pass a smaller font 
size to RFS, but then RFS won't dynamically scale this font size. For example: `responsive-font-size(19)` will trigger 
dynamic rescaling, with `responsive-font-size(10)` it will just stay `10px` all the time.  
*Default value: `1rem`*

**$rfs-font-size-unit:** (string)  
The font size will be rendered in this unit. Possible units are `px` and `rem`.  
*Default value: `rem`*

**$rfs-breakpoint:** (in `px`, `em` or `rem`)  
Above this breakpoint, the font size will be equal to the font size you passed to the mixin; below the breakpoint, the 
font size will dynamically scale.  
*Default value: `1200px`*

**$rfs-breakpoint-unit:** (string)  
The width of `$rfs-breakpoint` will be rendered in this unit. Possible units are `px`, `em` and `rem`.  
*Default value: `px`*

**$rfs-factor:** (number)  
This value determines the strength of font size resizing. The higher `$rfs-factor`, the less difference there is between
font sizes on small screens. The lower `$rfs-factor`, the less influence RFS has, which results in bigger font sizes for
small screens. `$rfs-factor` must me greater than 1, setting it to 1 will disable dynamic rescaling.  
*Default value: `5`*

**$rfs-two-dimensional** (Boolean)  
Enabling the two dimensional media queries will determine the font size based on the smallest side of the screen with
`vmin`. This prevents the font size from changing if the device toggles between portrait and landscape mode.  
*Default value: `false`*

**$rfs-generate-disable-classes** (Boolean)  
When the the disable classes are generated you can add the `.disable-responsive-font-size` class to an element to 
disable responsive font sizes for the element and its child elements. If you don't use this, it's better to set this to 
`false` to prevent the generation of unused css. This doesn't apply on font-sizes which are inherited from parents.  
*Default value: `true`*

## !important
By setting a second parameter to true, `!important` is added after the font-size value.

```scss
.label {
  @include responsive-font-size(2.5rem, true);
}
```

CSS:
```css
.label,
.disable-responsive-font-size .label,
.label.disable-responsive-font-size {
  font-size: 2.5rem !important;
}

@media (max-width: 1200px) {
  .label {
    font-size: calc(1.3rem + 1.6vw) !important;
  }
}
```

## But this generates a lot of css?
True. But with gzip or other compression enabled, the difference in file size is barely noticeable due the high amount
of repetitive strings. If you really want to minimize the amount of generated css, setting 
`$rfs-generate-disable-classes` to `false` can make a difference (and lowers the global specificity). 

## Known issues
Safari doesn't recalculate the value of vw in a calc()-function for font-sizes in iframes if the min-width, max-width or
width is not set in vw after the iframe is resized (edge case, but this is the case for Codepen demo's). Adding this 
line will solve this (dirty fix):
```css
_::-webkit-full-page-media, _:future, :root * {min-width: 0vw;}
```

## Best practices
- Remember to set RFS on your font size of your `html` or `body`, otherwise some text may not dynamically rescale (if 
`$rfs-minimum-font-size` is decreased). Note that setting RFS on `html` can influence the value of `rem`.
- Always set your line-heights relative (in `em` or unitless).
- More tips and tricks with examples can be found
[on this article](https://medium.com/@martijn.cuppens/magic-font-resizing-with-rfs-b5d781296dd6).

##  Demos
- [Simple Codepen Demo](http://codepen.io/MartijnCuppens/pen/ZBjdMy)
- [RFS in bootstrap demo](http://martijncuppens.github.io/rfs)