I had an idea for caching SVG paths. Not the usual kind of async request caching of remote SVGs, but local re-use of DOM elements that have already rendered.

SVG's [`<use>`](https://developer.mozilla.org/en-US/docs/Web/SVG/Element/use) element allows re-use of an existing DOM element, without manually duplicating the node. It works like this:

```html

<svg>
  <circle id="circle" cx="5" r="5" fill="black" />
</svg>


<svg>
  <use href="#circle" />
</svg>


```

## Setup

When using an icon set like [Feather](https://feathericons.com/) in React, I prefer to use a higher-order component (HOC) and a generic `Icon` component to render each icon with consistent properties. We'll use this HOC to demonstrate SVG caching:

```jsx
import { memo } from 'react'

const withIcon = (icon, opts) => {
  const Icon = props => {
    const { size = 24, color = 'currentColor' } = props

    return (
      <svg
        viewBox="0 0 24 24"
        width={size}
        height={size}
        stroke="currentColor"
        style={{
          color
        }}
        dangerouslySetInnerHTML={{
          __html: icon
        }}
      />
    )
  }

  return memo(Icon)
}

export default withIcon
```

Each icon is simply the SVG contents wrapped with the HOC:

```jsx
const ArrowLeft = withIcon('<path d="M21 12H3m0 0l6.146-6M3 12l6.146 6" />')
```

## Caching

We'll use React context to add an icon cache. First, create a new context and the appropriate hook to access it:

```jsx
export const IconCache = React.createContext(null)
export const useIconCache = () => React.useContext(IconCache)
```

Setup the provider at the application root. The cache will be a plain, empty object where each key is the icon string and each value is the cached id.

```jsx raw
const App = () => (
  <IconCache.Provider value={{}}>{/* ... */}</IconCache.Provider>
)
```

Inside of `Icon`, read the cache from context and check if this icon has a cached id. If not, generate the new id and add it to the cache:

```jsx
const cache = useIconCache()

let cachedId = cache[icon]

if (!cachedId) {
  cachedId = `icon-` + hash(icon).toString(16)
  cache[icon] = cachedId
}
```

Generate a stable id by hashing the icon using the [fnv1a](https://en.wikipedia.org/wiki/Fowler%E2%80%93Noll%E2%80%93Vo_hash_function) <a href="#footnote"><sup>1</sup></a> algorithm (commonly used in CSS-in-JS libraries) and then converting it to hexadecimal for a smaller string:

```jsx
import hash from 'fnv1a'
```

If we have a cached id, we can render the `<use>` tag instead of inserting the entire icon again. If this icon has not rendered before, wrap it in a group tag and attach the unique id.

```jsx highlight=11-13
return (
  <svg
    viewBox="0 0 24 24"
    width={size}
    height={size}
    stroke="currentColor"
    style={{
      color
    }}
    dangerouslySetInnerHTML={{
      __html: cachedId
        ? `<use href="#${cachedId}" />`
        : `<g id="${id}">${icon}</g>`
    }}
  />
)
```

## Conclusion

Here's our new `withIcon` HOC with caching:

```jsx highlight=2,4-5,10,12,13,15-18,30-32
import { memo } from 'react'
import hash from 'fnv1a'

export const IconCache = React.createContext({})
export const useIconCache = () => React.useContext(IconCache)

const withIcon = icon => {
  const Icon = props => {
    const { size = 24, color = 'currentColor' } = props
    const cache = useIconCache()

    const cachedId = cache[icon]
    let id

    if (!cachedId) {
      id = 'icon-' + hash(icon).toString(16)
      cache[icon] = id
    }

    return (
      <svg
        viewBox="0 0 24 24"
        width={size}
        height={size}
        stroke="currentColor"
        style={{
          color
        }}
        dangerouslySetInnerHTML={{
          __html: cachedId
            ? `<use href="#${cachedId}" />`
            : `<g id="${id}">${icon}</g>`
        }}
      />
    )
  }

  return memo(Icon)
}

export default withIcon
```

Rendering the same icon multiple times will reuse existing DOM elements, decreasing the size of your HTML:

```jsx raw
/* React */

<IconCache.Provider value={{}}>
  <ArrowLeft />
  <ArrowLeft />
  <ArrowLeft />
</IconCache.Provider>

/* HTML Output:
  <svg>
    <g id="icon-dacb5a47"><path d="M21 12H3m0 0l6.146-6M3 12l6.146 6" /></g>
  </svg>

  <svg>
    <use href="#icon-dacb5a47" />
  </svg>

  <svg>
    <use href="#icon-dacb5a47" />
  </svg>
*/
```

In this example, the cached version is about 40% fewer characters!

You can still customize each icon, because the props apply to the outer svg element and don't involve the inner elements at all:

```jsx
<ArrowLeft />
<ArrowLeft size={30} color="blue" />
<ArrowLeft size={50} color="red" />
```

---

Here's a [live demo](https://svgcache.vercel.app) and the [demo source code](https://github.com/pacocoursey/svgcache).

<div id="footnote"></div>

1. You don't have to use fnv1a, any stable id generation technique will work. Just make sure it's consistent between server and client to avoid hydration mismatch.

Smaller DOM size by reusing existing SVG icons

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