Question

Does anyone have a more sophisticated solution/library for truncating strings with JavaScript and putting an ellipsis on the end, than the obvious one:

if (string.length > 25) {
  string = string.substring(0, 24) + "...";
}

Answer 1

Essentially, you check the length of the given string. If it’s longer than a given length n, clip it to length n (substr or slice) and add html entity … (…) to the clipped string.

Such a method looks like

function truncate(str, n){
  return (str.length > n) ? str.slice(0, n-1) + '…' : str;
};

If by ‘more sophisticated’ you mean truncating at the last word boundary of a string then you need an extra check.
First you clip the string to the desired length, next you clip the result of that to its last word boundary

function truncate( str, n, useWordBoundary ){
  if (str.length <= n) { return str; }
  const subString = str.slice(0, n-1); // the original check
  return (useWordBoundary 
    ? subString.slice(0, subString.lastIndexOf(" ")) 
    : subString) + "&hellip;";
};

You can extend the native String prototype with your function. In that case the str parameter should be removed and str within the function should be replaced with this:

String.prototype.truncate = String.prototype.truncate || 
function ( n, useWordBoundary ){
  if (this.length <= n) { return this; }
  const subString = this.slice(0, n-1); // the original check
  return (useWordBoundary 
    ? subString.slice(0, subString.lastIndexOf(" ")) 
    : subString) + "&hellip;";
};

More dogmatic developers may chide you strongly for that ("Don’t modify objects you don’t own". I wouldn’t mind though). [edit 2023] A method to extend the String without tampering with its prototype may be to use a Proxy. See this stackblitz snippet.

An approach without extending the String prototype is to create
your own helper object, containing the (long) string you provide
and the beforementioned method to truncate it. That’s what the snippet
below does.

const LongstringHelper = str => {
  const sliceBoundary = str => str.substr(0, str.lastIndexOf(" "));
  const truncate = (n, useWordBoundary) => 
        str.length <= n ? str : `${ useWordBoundary 
          ? sliceBoundary(str.slice(0, n - 1))
          : str.slice(0, n - 1)}&hellip;`;
  return { full: str,  truncate };
}; 
const longStr = LongstringHelper(`Lorem ipsum dolor sit amet, consectetur 
adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore 
magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation 
ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute 
irure dolor in reprehenderit in voluptate velit esse cillum dolore 
eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non 
proident, sunt in culpa qui officia deserunt mollit anim id est laborum`);

const plain = document.querySelector("#resultTruncatedPlain");
const lastWord = document.querySelector("#resultTruncatedBoundary");
plain.innerHTML = 
  longStr.truncate(+plain.dataset.truncateat, !!+plain.dataset.onword);
lastWord.innerHTML = 
  longStr.truncate(+lastWord.dataset.truncateat, !!+lastWord.dataset.onword);
document.querySelector("#resultFull").innerHTML = longStr.full;

body {
  font: normal 12px/15px verdana, arial;
}

p {
  width: 450px;
}

#resultTruncatedPlain:before {
  content: 'Truncated (plain) n='attr(data-truncateat)': ';
  color: green;
}

#resultTruncatedBoundary:before {
  content: 'Truncated (last whole word) n='attr(data-truncateat)': ';
  color: green;
}

#resultFull:before {
  content: 'Full: ';
  color: green;
}

<p id="resultTruncatedPlain" data-truncateat="120" data-onword="0"></p>
<p id="resultTruncatedBoundary" data-truncateat="120" data-onword="1"></p>
<p id="resultFull"></p>

Finally, you can use css only to truncate long strings in HTML nodes. It gives you less control, but may well be viable solution.

body {
  font: normal 12px/15px verdana, arial;
  margin: 2rem;
}

.truncate {
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
  width: 30vw;
}

.truncate:before{
  content: attr(data-longstring);
}

.truncate:hover::before {
  content: attr(data-longstring);
  width: auto;
  height: auto;
  overflow: initial;
  text-overflow: initial;
  white-space: initial;
  background-color: white;
  display: inline-block;
}

<div class="truncate" data-longstring="Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum."></div>