Dynamically add <link /> tag to your HTML page

add-link-tag.ts

type NewLinkTagStatus = 'ALREADY_EXISTS' | 'CREATED' | 'ERROR'
type NewLinkTagArgs = Pick<HTMLLinkElement, 'href' | 'rel' | 'type'> & Partial<HTMLLinkElement>

export function newLinkTag(attributes: NewLinkTagArgs): NewLinkTagStatus {
  // these two attributes are required
  const { href, rel } = attributes
  if (!href || !rel) {
    console.error('newLinkTag: href and rel are required attributes')
    return 'ERROR'
  }
  // check if tag already exists
  const tagExists = document.querySelector(`link[href="${href}"]`) as HTMLLinkElement | null
  if (tagExists) return 'ALREADY_EXISTS'

  // remove props/attributes that are null/undefined
  const validAttributes = JSON.parse(JSON.stringify(attributes))
  // create new <link /> element and add attributes
  let newElement = document.createElement('link')
  newElement = Object.assign(newElement, validAttributes)

  /**
   * Next 5 lines are optional.
   * For organizational purposes, I like having a <meta name="EXTERNAL_CSS" /> tag in
   * my head that I can add my external CSS under to keep things organized.
   */
  let insertLocation = document.querySelector('meta[name="EXTERNAL_CSS"]') as HTMLMetaElement | null
  if (!insertLocation) {
    insertLocation = document.createElement('meta') as HTMLMetaElement
    Object.assign(insertLocation, { name: 'EXTERNAL_CSS' })
  }

  // insert new <link /> element after <meta name="EXTERNAL_CSS" />
  insertLocation.insertAdjacentElement('afterend', newElement)
  return 'CREATED'
}

// Usage
const linkTag = newLinkTag({
  rel: 'stylesheet',
  // this is the stylesheet.css below
  href: 'https://gist.githubusercontent.com/o-az/eb41ae192797f424f8053ffad98cc10b/raw/306a5b678779e79b842b09e54f166a7e349bb9a6/stylesheet.css',
  type: 'text/css',
  title: 'prism-synthwave84',
})

console.log(`prism-synthwave84 link tag: ${linkTag}`)

stylesheet.css

/*
 * Synthwave '84 Theme originally by Robb Owen [@Robb0wen] for Visual Studio Code
 * Demo: https://marc.dev/demo/prism-synthwave84
 *
 * Ported for PrismJS by Marc Backes [@themarcba]
 */

code[class*="language-"],
pre[class*="language-"] {
	color: #f92aad;
	text-shadow: 0 0 2px #100c0f, 0 0 5px #dc078e33, 0 0 10px #fff3;
	background: none;
	font-family: Consolas, Monaco, 'Andale Mono', 'Ubuntu Mono', monospace;
	font-size: 1em;
	text-align: left;
	white-space: pre;
	word-spacing: normal;
	word-break: normal;
	word-wrap: normal;
	line-height: 1.5;

	-moz-tab-size: 4;
	-o-tab-size: 4;
	tab-size: 4;

	-webkit-hyphens: none;
	-moz-hyphens: none;
	-ms-hyphens: none;
	hyphens: none;
}

/* Code blocks */
pre[class*="language-"] {
	padding: 1em;
	margin: .5em 0;
	overflow: auto;
}

:not(pre) > code[class*="language-"],
pre[class*="language-"] {
	background-color: transparent !important;
	background-image: linear-gradient(to bottom, #2a2139 75%, #34294f);
}

/* Inline code */
:not(pre) > code[class*="language-"] {
	padding: .1em;
	border-radius: .3em;
	white-space: normal;
}

.token.comment,
.token.block-comment,
.token.prolog,
.token.doctype,
.token.cdata {
	color: #8e8e8e;
}

.token.punctuation {
	color: #ccc;
}

.token.tag,
.token.attr-name,
.token.namespace,
.token.number,
.token.unit,
.token.hexcode,
.token.deleted {
	color: #e2777a;
}

.token.property,
.token.selector {
	color: #72f1b8;
	text-shadow: 0 0 2px #100c0f, 0 0 10px #257c5575, 0 0 35px #21272475;
}

.token.function-name {
	color: #6196cc;
}

.token.boolean,
.token.selector .token.id,
.token.function {
	color: #fdfdfd;
	text-shadow: 0 0 2px #001716, 0 0 3px #03edf975, 0 0 5px #03edf975, 0 0 8px #03edf975;
}

.token.class-name {
	color: #fff5f6;
	text-shadow: 0 0 2px #000, 0 0 10px #fc1f2c75, 0 0 5px #fc1f2c75, 0 0 25px #fc1f2c75;
}

.token.constant,
.token.symbol {
	color: #f92aad;
	text-shadow: 0 0 2px #100c0f, 0 0 5px #dc078e33, 0 0 10px #fff3;
}

.token.important,
.token.atrule,
.token.keyword,
.token.selector .token.class,
.token.builtin {
	color: #f4eee4;
	text-shadow: 0 0 2px #393a33, 0 0 8px #f39f0575, 0 0 2px #f39f0575;
}

.token.string,
.token.char,
.token.attr-value,
.token.regex,
.token.variable {
	color: #f87c32;
}

.token.operator,
.token.entity,
.token.url {
	color: #67cdcc;
}

.token.important,
.token.bold {
	font-weight: bold;
}

.token.italic {
	font-style: italic;
}

.token.entity {
	cursor: help;
}

.token.inserted {
	color: green;
}