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Highlight syntax in contenteditable

I have a contenteditable div I want users to type in. When users type inside the box with onkeyup, I activate a function that changes the color of certain characters:

var lTags = fixedCode.innerHTML.replace(/&lt;/gi, "<span style='color:gold;'>&lt;</span>");
var rTags = lTags.replace(/&gt;/gi, "<span style='color:gold'>&gt;</span>");
fixedCode.innerHTML = rTags;

What this code does is it takes every < sign and every > sign and turns it into a gold color. However, when I do this, I am no longer able to type words into the contenteditable box since the box refreshes itself every time I press a key. 

function checkIt(code) {
   var fixedCode = document.getElementById(code);
   var lTags = fixedCode.innerHTML.replace(/&lt;/gi, "<span style='color:gold;'>&lt;</span>");
   var rTags = lTags.replace(/&gt;/gi, "<span style='color:gold'>&gt;</span>");
   fixedCode.innerHTML = rTags;
}
<div id="box" contenteditable="true" onkeyup="checkIt(this.id);">See for yourself</div>

To see for yourself, try typing any HTML tag in the box. First of all, why does it change the color of the left < of a tag but not the right part of the tag >? And how can I actually type inside the box without deleting the color-changing stuff. I’ve seen similar questions, but the answers were Jquery. I do not want to use JQUERY!

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Answer

I was too lazy to go hardcore with JavaScript and one idea that popped my mind was to use

  • two overlaying DIV elements
  • the overlaying contenteditable has transparent text but visible caret!
  • the underlaying DIV is the one that shows the colorful syntax highlighted content

PROS

  • The pros about this technique is that you always keep (in the contenteditable DIV) the unchanged content in its original state.

CONS

  • On every keystroke we parse all over again the same content, and as it gets bigger it might be a performance killer as your replacement list grows O(n)

For a beautiful read on optimization head to VSCode Optimizations in Syntax Highlighting

Basic example:

const highLite = (el) => {
  el.previousElementSibling.innerHTML = el.innerHTML
     .replace(/(&lt;|&gt;)/g, "<span class='hl_angled'>$1</span>")
     .replace(/({|})/g, "<span class='hl_curly'>$1</span>");
};

document.querySelectorAll("[contenteditable]").forEach(el => {
  el.addEventListener("input", () => highLite(el));
  highLite(el);
});
body{margin:0; font:14px/1 sans-serif;}

.highLite{
  border: 1px solid #888;
  position: relative;
}

.highLite_colors,
.highLite_editable {
  padding: 16px;
}

/* THE UNDERLAYING ONE WITH COLORS */
.highLite_colors {
  position: absolute;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0; 
  user-select: none;
}

/* THE OVERLAYING CONTENTEDITABLE WITH TRANSPARENT TEXT */
.highLite_editable {
  position: relative;
  color: transparent; /* Make text invisible */
  caret-color: black; /* But keep caret visible */
}

.hl_angled{ color: turquoise; }
.hl_curly{ color: fuchsia; }
Try to type some angled &lt; &gt; or curly { } brackets
<div class="highLite">
  <div class="highLite_colors">Type &lt;here&gt; {something}</div>
  <div class="highLite_editable" contenteditable>Type &lt;here&gt; {something}</div>
</div>

Advanced example:

(use at your own risk this was my coffee-time playground)

const lang = {
  js: {
    equa: /(b=b)/g,
    quot: /(('.*?')|(".*?")|(".*?(?<!\)")|('.*?(?<!\)')|`)/g,
    comm: /((/*([^*]|[rn]|(*+([^*/]|[rn])))**+/)|(//.*))/g,
    logi: /(%=|%|-|+|*|&amp;{1,2}||{1,2}|&lt;=|&gt;=|&lt;|&gt;|!={1,2}|={2,3})/g,
    numb: /(d+(.d+)?(ed+)?)/g,
    func: /(?<=^|s*)(async|await|console|alert|Math|Object|Array|String|class(?!s*=)|function)(?=b)/g,
    decl: /(?<=^|s*)(var|let|const)/g,
    pare: /((|))/g,
    squa: /([|])/g,
    curl: /({|})/g,
  },
  html: {
    tags: /(?<=&lt;(?:/)?)(w+)(?=s|&gt;)/g,
    // Props order matters! Here I rely on "tags"
    // being already applied in the previous iteration
    angl: /(&lt;/?|&gt;)/g,
    attr: /((?<=<i class=html_tags>w+</i>)[^<]+)/g,
  }
};

const highLite = el => {
  const dataLang = el.dataset.lang; // Detect "js", "html", "py", "bash", ...
  const langObj = lang[dataLang]; // Extract object from lang regexes dictionary
  let html = el.innerHTML;
  Object.keys(langObj).forEach(function(key) {
    html = html.replace(langObj[key], `<i class=${dataLang}_${key}>$1</i>`);
  });
  el.previousElementSibling.innerHTML = html; // Finally, show highlights!
};

const editors = document.querySelectorAll(".highLite_editable");
editors.forEach(el => {
  el.contentEditable = true;
  el.spellcheck = false;
  el.autocorrect = "off";
  el.autocapitalize = "off";
  el.addEventListener("input", () => highLite(el));
  highLite(el); // Init!
});
* {margin: 0; box-sizing: boder-box;}

body {
  font: 14px/1.4 sans-serif;
  background: hsl(220, 16%, 16%);
  color: #fff;
  padding: 16px;
}

#editor {
  display: flex;
}

h2 {
  padding: 16px 0;
  font-weight: 200;
  font-size: 14px;
}

.highLite {
  position: relative;
  background: hsl(220, 16%, 14%);
}

.highLite_colors,
.highLite_editable {
  padding: 16px;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  white-space: pre-wrap;
  font-family: monospace;
  font-size: 13px;
}

/* THE OVERLAYING CONTENTEDITABLE WITH TRANSPARENT TEXT */
.highLite_editable {
  position: relative;
  color: transparent; /* Make text invisible */
  caret-color: hsl( 50, 75%, 70%); /* But keep caret visible */
}
.highLite_editable:focus {
  outline: 1px solid hsl(220, 16%, 19%);
}
.highLite_editable::selection {
  background: hsla( 0, 0%, 90%, 0.2);
}

/* THE UNDERLAYING ONE WITH HIGHLIGHT COLORS */
.highLite_colors {
  position: absolute;
  user-select: none;
}

.highLite_colors i {
  font-style: normal;
}

/* JS */
i.js_quot { color: hsl( 50, 75%, 70%); }
i.js_decl { color: hsl(200, 75%, 70%); }
i.js_func { color: hsl(300, 75%, 70%); }
i.js_pare { color: hsl(210, 75%, 70%); }
i.js_squa { color: hsl(230, 75%, 70%); }
i.js_curl { color: hsl(250, 75%, 70%); }
i.js_numb { color: hsl(100, 75%, 70%); }
i.js_logi { color: hsl(200, 75%, 70%); }
i.js_equa { color: hsl(200, 75%, 70%); }
i.js_comm { color: hsl(200, 10%, 45%); font-style: italic; }
i.js_comm > * { color: inherit; }

/* HTML */
i.html_angl { color: hsl(200, 10%, 45%); }
i.html_tags { color: hsl(  0, 75%, 70%); }
i.html_attr { color: hsl(200, 74%, 70%); }
<h2>HTML</h2>
<div class="highLite">
    <div class="highLite_colors"></div>
    <div class="highLite_editable" data-lang="html">&lt;h2 class="head"&gt;
TODO: HTML is for &lt;b&gt;homework&lt;/b&gt;
&lt;/h2&gt;</div>
</div>

<h2>JAVASCRIPT</h2>
<div class="highLite">
    <div class="highLite_colors"></div>
    <div class="highLite_editable" data-lang="js">// Type some JavaScript here

const arr = ["high", 'light'];
let n = 2.1 * 3;
if (n &lt; 10) {
  console.log(`${n} is &lt;= than 10`);
}
function casual(str) {
  str = str || "non"sense";
  alert("Just a casual"+ str +", still many TODOs");
}
casual (arr.join('') +" idea!");

/**
* The code is a proof of concept and far from
* perfect. You should never use regex but create or use a parser.
* Meanwhile, play with it and improve it!
*/</div>
</div>

TODO
given this basic idea, some TODOs I left for the reader:

  • when the DIV we’re editing receives scrollbars, update accordingly the scroll position for the sibling DIV using JavaScript.
  • instead of using Regex, use or create a proper parser to perform lexical analysis (tokenization) or syntactic analysis (parsing) for the specific languages you want to support in your syntax highlighter.
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