Detecting unminified code

minify-detect.js

// https://hg.mozilla.org/mozilla-central/rev/2f9043292e63
// Used to detect minification for automatic pretty printing
const SAMPLE_SIZE = 30; // no of lines
const INDENT_COUNT_THRESHOLD = 20; // percentage

function isMinified (str) {
  let isMinified;
  let lineEndIndex = 0;
  let lineStartIndex = 0;
  let lines = 0;
  let indentCount = 0;

  // Strip comments.
  str = str.replace(/\/\*[\S\s]*?\*\/|\/\/(.+|\n)/g, "");

  while (lines++ < SAMPLE_SIZE) {
    lineEndIndex = str.indexOf("\n", lineStartIndex);
    if (lineEndIndex == -1) {
       break;
    }
    if (/^\s+/.test(str.slice(lineStartIndex, lineEndIndex))) {
      indentCount++;
    }
    lineStartIndex = lineEndIndex + 1;
  }
  isMinified = ((indentCount / lines ) * 100) < INDENT_COUNT_THRESHOLD;

  return isMinified;
};

FYI: The one from Google DevTools is much faster:
https://github.com/ChromeDevTools/devtools-frontend/blob/main/front_end/models/text_utils/TextUtils.ts#L336

small not minified file (5kb)

$ node benchmark.js

isMinified#FF x 213,571 ops/sec ±0.42% (89 runs sampled)
isMinified#Google x 1,196,225 ops/sec ±0.22% (91 runs sampled)
Fastest is isMinified#Google

angular.min.js (173Kb)

$ node benchmark.js

isMinified#FF x 3,861 ops/sec ±0.33% (92 runs sampled)
isMinified#Google x 7,116,082 ops/sec ±0.16% (92 runs sampled)
Fastest is isMinified#Google

angular.js (1.3Mb)

node benchmark.js

isMinified#FF x 460 ops/sec ±7.02% (80 runs sampled)
isMinified#Google x 850,793 ops/sec ±0.45% (93 runs sampled)
Fastest is isMinified#Google

But not as reliable, when there is eval() usage with unminified code inside FF version says it's not minified (which is truth) while the one from google dev tools says it is. But that's rather an edge case scenario