Sprockets removes sourceMappingURL and sourceURL from module in browser

[TomaszPanek]

Expected behavior

First of all I'm upgrading rails from 4 to 6 version.
I have a separate fronted project that's based on webpack and as a result it produces js output file. Let's name it ui.js
This file is later taken by sprockets using //=require 'ui'
On my current stack (rails 4.2.11.3, sprockets 2.12.5, uglifier 4.1.20) everything was working fine, I was able to serve this file to client without any errors.
Part of the ui.js file that works in rails 4(before update):

eval("\nvar $export = __webpack_require__(/*! ./_export */ \"./node_modules/core-js/modules/_export.js\");\nvar $map = __webpack_require__(/*! ./_array-methods */ \"./node_modules/core-js/modules/_array-methods.js\")(1);\n\n$export($export.P + $export.F * !__webpack_require__(/*! ./_strict-method */ \"./node_modules/core-js/modules/_strict-method.js\")([].map, true), 'Array', {\n  // 22.1.3.15 / 15.4.4.19 Array.prototype.map(callbackfn [, thisArg])\n  map: function map(callbackfn /* , thisArg */) {\n    return $map(this, callbackfn, arguments[1]);\n  }\n});\n//# sourceURL=[module]\n//# sourceMappingURL=data:application/json;charset=utf-8;base64,eyJ2ZXJzaW9uIjozLCJzb3VyY2VzIjpbIndlYnBhY2s6Ly8vLi9ub2RlX21vZHVsZXMvY29yZS1qcy9tb2R1bGVzL2VzNi5hcnJheS5tYXAuanM/NmQ2NyJdLCJuYW1lcyI6W10sIm1hcHBpbmdzIjoiQUFBYTtBQUNiLGNBQWMsbUJBQU8sQ0FBQyw0REFBVztBQUNqQyxXQUFXLG1CQUFPLENBQUMsMEVBQWtCOztBQUVyQyxpQ0FBaUMsbUJBQU8sQ0FBQywwRUFBa0I7QUFDM0Q7QUFDQTtBQUNBO0FBQ0E7QUFDQSxDQUFDIiwiZmlsZSI6Ii4vbm9kZV9tb2R1bGVzL2NvcmUtanMvbW9kdWxlcy9lczYuYXJyYXkubWFwLmpzLmpzIiwic291cmNlc0NvbnRlbnQiOlsiJ3VzZSBzdHJpY3QnO1xudmFyICRleHBvcnQgPSByZXF1aXJlKCcuL19leHBvcnQnKTtcbnZhciAkbWFwID0gcmVxdWlyZSgnLi9fYXJyYXktbWV0aG9kcycpKDEpO1xuXG4kZXhwb3J0KCRleHBvcnQuUCArICRleHBvcnQuRiAqICFyZXF1aXJlKCcuL19zdHJpY3QtbWV0aG9kJykoW10ubWFwLCB0cnVlKSwgJ0FycmF5Jywge1xuICAvLyAyMi4xLjMuMTUgLyAxNS40LjQuMTkgQXJyYXkucHJvdG90eXBlLm1hcChjYWxsYmFja2ZuIFssIHRoaXNBcmddKVxuICBtYXA6IGZ1bmN0aW9uIG1hcChjYWxsYmFja2ZuIC8qICwgdGhpc0FyZyAqLykge1xuICAgIHJldHVybiAkbWFwKHRoaXMsIGNhbGxiYWNrZm4sIGFyZ3VtZW50c1sxXSk7XG4gIH1cbn0pO1xuIl0sInNvdXJjZVJvb3QiOiIifQ==\n//# sourceURL=webpack-internal:///./node_modules/core-js/modules/es6.array.map.js\n")

Actual behavior

Problems occurred when I updated sprockets from 2 to latest 4.2.1.
Rails/sprockets servers above code without sourceURL and sourceMappingURL thus I'm getting error in browser console and ui crashes.

eval('\nvar $export = __webpack_require__(/*! ./_export */ "./node_modules/core-js/modules/_export.js");\nvar $map = __webpack_require__(/*! ./_array-methods */ "./node_modules/core-js/modules/_array-methods.js")(1);\n\n$export($export.P + $export.F * !__webpack_require__(/*! ./_strict-method */ "./node_modules/core-js/modules/_strict-method.js")([].map, true), \'Array\', {\n  // 22.1.3.15 / 15.4.4.19 Array.prototype.map(callbackfn [, thisArg])\n  map: function map(callbackfn /* , thisArg */) {\n    return $map(this, callbackfn, arguments[1]);\n  }\n});\n//# sourceURL=[module]\n.js\n')

So the missing part is:

//# sourceMappingURL=data:application/json;charset=utf-8;base64,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\n//# sourceURL=webpack-internal:///./node_modules/core-js/modules/es6.array.map

What's interesting this problem happens only for this one module, others are served 'normally'

To resolve this issue, I was playing around with different options

• changing uglifier options like mangle: { eval: false} etc. - however I'm not sure which one could be responsible for it.
• switch to terser
• turning on/off assets.debug, assets.compress, assets.compile

Unfortunately none of above resolve this issue. Thus I'm seeking for some advice :)

System configuration

• sprockets 4.2.1
• uglifier 4.1.20
• sprockets-rails 3.4.2
• Ruby version 2.7.3
• rails 6.1.7.7