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Awesome Open Source


Perfect vertical rhythm for typography and more in CSS-in-JS

ShevyJS takes the concepts of the original Shevy and makes them available for CSS-in-JS systems. Shevy will do all the math required to keep your typography (and more) on your design system's baseline.


Shevy is available as a module from npm:

npm install shevyjs

Or, with Yarn:

yarn add shevyjs


Be sure that you get the shevyjs module, otherwise you might accidentally import the Sass version of this library instead.


ShevyJS is not designed for any particular framework or CSS-in-JS solution. It can be used anywhere you can use JavaScript. The following example will use React, but you should be able to apply these concepts to your needs with ease.

import React from 'react'
import createShevy from 'shevyjs' // const Shevy = require('shevyjs').default if using CommonJS

const shevy = createShevy() // factory function for creating Shevy objects
const { h1, content } = shevy // Destructures the styles for h1 and content-based tags

const MyComponent = () => (
    <h1 style={h1}>ShevyJS</h1>
    <p style={content}>Shevy's not just for Sass anymore.</p>

export default MyComponent

Creating a Shevy object will generate a set of properties that you can use for your styles in your components. Typically, you'll create a single Shevy object and utilize that throughout your application, but it's certainly possible to make localized shevys for your use cases.



createShevy: (options?: ShevyOptions = {}) => Shevy

createShevy is a factory function for creating Shevy objects. Invoking it without options will utilize the defaultOptions discussed below. createShevy can receive an object of ShevyOptions to be merged with the defaults.

The most common use would be something like the following:

// shevy.js
import createShevy from 'shevyjs'

const shevy = createShevy()
export default shevy

// I like to alias the baseSpacing function to a shorthand for my projects
// since it is commonly used for paddings and margins
export const bs = shevy.baseSpacing

Default Options

ShevyJS comes with a set of defaults that can be easily overwritten. To overwrite any of these options, pass an options object into Shevy() at instantiation. Your options object will be merged with the default options, so you can declare as many or as few of the options as you would like.

Here are the defaults:

const defaultOptions = {
  baseFontSize: '16px',
  baseLineHeight: 1.5,
  fontScale: [3, 2.5, 2, 1.5, 1.25, 1],
  includeMarginBottom: true,
  proximity: null,
  precision: null,

Below is a description of what each option does for ShevyJS.


baseFontSize: string

This is the size you want to base your typography on. Typically, this will be the smallest or default size of your typography. It is possible to use fontScales in a way that would allow for headings to be smaller than your default font size, but it's unlikely you'll use ShevyJS this way.


baseLineHeight: number

This is used to determine the line height calculations in Shevy. It is required that this value be unitless and a number.

Line heights for your headings will be based on multiples of baseLineHeight / 2. Half increments of your baseLineHeight, while strictly speaking will shift your rhythm, are typically more aesthetically pleasing that strictly using whole increments. This prevents fontSizes slightly larger than a baseLineHeight multiple from becoming excessively large.

Example: a fontSize of 50px and a baseLineHeight of 48px doesn't result in a 96px lineHeight, but rather a 72px lineHeight (48/2 gives us a half increment of 24. 24*3 is the first size greater than 50, thus 72px).


fontScale: number[]

This is an array, of max length 6 (any values beyond the 6th will be trimmed), that is used to generate the h1 through h6 styles. Each value should be a number, that will be multiplied by the baseFontSize to generate the font size for that heading.

Font scale presets are available based on If baseFontScale is a string, it will attempt to match one of the following presets by key. If a match does not exist, an error will be thrown. The presets are:

  majorSecond: [1.802, 1.602, 1.424, 1.266, 1.125, 1],
  minorThird: [2.488, 2.074, 1.728, 1.44, 1.2, 1],
  majorThird: [3.052, 2.441, 1.953, 1.563, 1.25, 1],
  perfectFourth: [4.209, 3.157, 2.369, 1.777, 1.333, 1],
  augmentedFourth: [5.653, 3.998, 2.827, 1.999, 1.414, 1]

You may supply fewer than 6 values to the fontScale. Doing so will result in the corresponding h* values return objects that look like:

  fontSize: undefined,
  lineHeight: undefined,
  marginBottom: undefined,

This API was chosen to ensure type-safety and convenience. h1 through h6 will always exist on a shevy object. You will not need conditionals to check for their existence. But perhaps your application never uses h4 through h6, and thus it's needless to define them. This gives you that flexibility.


includeMarginBottom: boolean

This determines whether a value will be set for the marginBottom of your style objects.


proximity: null | number

It is often more aesthetically pleasing to make your margins smaller than your baseline would typically warrant. This is due to the fact that line height in CSS is applied above and below the fontSize. This results in half the extra line height sitting below the text. Adding spacing beyond this, while mathematically correct, may not be the look you are going for.

By default, proximity is set to null, but setting it to a number will tweak the spacings by that percentage.


precision: null | number

By default, precision is set to null, but setting it to a number will result in values that do not exceed that number of places after the decimal. Example: 1.23456 with a precision of 4 becomes 1.2346.


Each Shevy object comes with a set of properties to use for your styles. Each property is a JavaScript object of the following styles:

  fontSize: string
  lineHeight: number | string
  marginBottom: string

There are cases (see fontScale) where these properties are undefined.

Here are the available properties:

  • h1, h2, h3, h4, h5, h6
  • body
  • content

h1 through h6 properties map to the results of calculating your options. Here is an example of one of these objects:

const shevy = createShevy()

console.log(shevy.h1) // { fontSize: '48px', lineHeight: 1, marginBottom: '24px' }

The body property is intended to go on the <body> tag selector and is ported over from the original Shevy. This may be less necessary in a component based JS system and might be deprecated in the future. Here is an example of the body object:

const shevy = createShevy()

console.log(shevy.body) // { fontSize: '16px', lineHeight: 1.5 }

The content tag is intended to be used for any base content level components. In the original Shevy, this was a mixin that directly applied styles to the <p>, <ol>, <ul>, and <pre> tags. Now in ShevyJS, you have much more freedom to apply these styles to whatever component you deem fit. Here is an example of the content object:

const shevy = createShevy()

console.log(shevy.content) // { fontSize: '16px', lineHeight: 1.5, marginBottom: '24px' }


Shevy has two methods that can be useful in your design system for creating distances that fall in line with your baseline grid.


lineHeightSpacing: (factor?: number = 1) => string

The lineHeightSpacing() method takes one argument, a number (which defaults to 1), and multiplies it with the result of the baseFontSize multiplied by the baseLineHeight.


baseSpacing: (factor?: number = 1) => string

The baseSpacing() method takes one argument, a number (which defaults to 1), and multiplies it with the result of baseFontSize multiplied by the baseLineHeight. It is additionally multiplies by proximity if it is not null.

Upgrade path from v1 to v2

There are a few differences between v1 to v2, so here's how to make those changes.

  • The default import is no longer a Shevy class constructor. It is the createShevy factory function.
- import Shevy from 'shevyjs'
+ import createShevy from 'shevyjs'
  • Replace instances of new Shevy(options) with createShevy(options)
- const shevy = new Shevy()
+ const shevy = createShevy()
  • Some options properties were renamed:

    • baseFontScale is now just fontScale
    • addMarginBottom is now includeMarginBottom
  • Some options properties were modified

    • precision and usePrecision are now just precision. precision is a nullable property now, replacing the need for two options.
    • proximity and proximityFactor are now just proximity. proximity is a nullable property now, replacing the need for two options.
const options = {
-  precision: 4,
-  usePrecision: true,
-  proximity: true,
-  proximityFactor: 0.85
+  precision: 4,
+  proximity: 0.85,
  • Several properties are no longer accessible on the shevy object

In v1, shevy was an instance of the Shevy class. Because of this, certain values were made properties of the class that really didn't need to be. An example would be this.baseFontScale. There isn't a good reason for this to need to be on the Shevy class.

With the conversion to a simple factory function, it was easy to keep certain values and functions private. This tidies up the exposed API to just the properties for styles, and the methods listed above.

This also improves the types for the project, which will likely improve your editor experience with faster Intellisense for shevy objects.

Example Uses of Shevy Methods

As Inline Styles

import React from 'react'
import createShevy from 'shevyjs'

const shevy = createShevy()
const { lineHeightSpacing: lhs, baseSpacing: bs } = shevy // Destructure and alias methods

const wrap = {
  marginBottom: lhs(2),

const box = {
  padding: bs(0.5),
  marginBottom: bs(),

const MyComponent = () => (
  <div style={wrap}>
    <div style={box}>Box 1</div>
    <div style={box}>Box 2</div>

With Styled Components

import React from 'react'
import styled from 'styled-components'
import createShevy from 'shevyjs'

const shevy = createShevy()
const {
  baseSpacing: bs,
  h1: { fontSize, lineHeight, marginBottom },
} = shevy

const Wrap = styled.div`
  padding: ${bs()};
  margin-bottom: ${bs(2)};

const Heading = styled.h1`
  font-size: ${fontSize};
  line-height: ${lineHeight};
  margin-bottom: ${marginBottom};

const MyComponent = () => (
    <Heading>Shevy with Styled Components!</Heading>

With Emotion

import createShevy from 'shevyjs'
import { css } from 'emotion'

const shevy = createShevy()
const { content } = shevy
const app = document.getElementById('root')
const myStyle = css`
  color: rebeccapurple;
  font-size: ${content.fontSize};

And Emotion with React:

import React from 'react'
import styled, { css } from 'emotion'
import Shevy from 'shevyjs'

const shevy = createShevy()
const {
  baseSpacing: bs,
  h1: { fontSize, lineHeight, marginBottom },
} = shevy

const Wrap = styled('div')`
  padding: ${bs()};
  margin-bottom: ${bs(2)};

const Heading = styled('h1')`
  font-size: ${fontSize};
  line-height: ${lineHeight};
  margin-bottom: ${marginBottom};

const MyComponent = () => (
    <Heading>Shevy with Emotion and React!</Heading>


Create a Spacer component to use with shevy (inspired by this Max Stoiber article):

import React from 'react'
import createShevy from 'shevyjs'

const shevy = createShevy()
const bs = shevy.baseSpacing

function Spacer({
  all = 0,
  horz = 0,
  vert = 0,
  top = 0,
  right = 0,
  bottom = 0,
  left = 0,
}) {
  const margins = {
    ...(all && { margin: bs(all) }),
    ...(horz && { marginLeft: bs(horz), marginRight: bs(horz) }),
    ...(vert && { marginTop: bs(vert), marginBottom: bs(vert) }),
    ...(top && { marginTop: bs(top) }),
    ...(right && { marginRight: bs(right) }),
    ...(bottom && { marginBottom: bs(bottom) }),
    ...(left && { marginLeft: bs(left) }),

  return <div style={margins}>{children}</div>

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