一、运行机制

有一个scss文件：

$color: red;
#test {
  color: $color;
}

sass-loader、css-loader、style-loader对上述内容的处理过程如下：

1、先经过style-loader的pitch方法（关于pitch方法我在vue-loader原理 (opens new window)的文章中有讲），将css/scss模块内容改为：

import API from "!../../../node_modules/style-loader/dist/runtime/injectStylesIntoStyleTag.js";
import domAPI from "!../../../node_modules/style-loader/dist/runtime/styleDomAPI.js";
import insertFn from "!../../../node_modules/style-loader/dist/runtime/insertBySelector.js";
import setAttributes from "!../../../node_modules/style-loader/dist/runtime/setAttributesWithoutAttributes.js";
import insertStyleElement from "!../../../node_modules/style-loader/dist/runtime/insertStyleElement.js";
import styleTagTransformFn from "!../../../node_modules/style-loader/dist/runtime/styleTagTransform.js";
import content, * as namedExport from "!!../../../node_modules/css-loader/dist/cjs.js!../../../node_modules/sass-loader/dist/cjs.js!./a.scss";

var options = {};

options.styleTagTransform = styleTagTransformFn;
options.setAttributes = setAttributes;

options.insert = insertFn.bind(null, "head");
    
options.domAPI = domAPI;
options.insertStyleElement = insertStyleElement;

var update = API(content, options);

export * from "!!../../../node_modules/css-loader/dist/cjs.js!../../../node_modules/sass-loader/dist/cjs.js!./a.scss";
export default content && content.locals ? content.locals : undefined;

简单来讲就是加载css内容，将其添加到一个style标签，并放到head中，简化版就是：

const content = require('!!../../../node_modules/css-loader/dist/cjs.js!../../../node_modules/sass-loader/dist/cjs.js!./a.scss')
const style = document.createElement('style');
style.innerHTML = content;
document.head.appendChild(style);

如何调试可以直观看到上述过程呢？打断点，在css被渲染前标记。下面是调试时画的流程图：

2、上述内容会经过内联loader处理，也就是sass-loader、css-loader依次处理，sass-loader主要是处理语法，处理结果为：

#test {
  color: red;
}

3、css-loader的话将css内容处理成如下模块，其实就是导出一个数组，数组元素内容为css内容，这个数组会被style-loader的运行时部分，也就是第1步利用，添加到head的style标签中。

// Imports
import ___CSS_LOADER_API_NO_SOURCEMAP_IMPORT___ from "../../../node_modules/css-loader/dist/runtime/noSourceMaps.js";
import ___CSS_LOADER_API_IMPORT___ from "../../../node_modules/css-loader/dist/runtime/api.js";
var ___CSS_LOADER_EXPORT___ = ___CSS_LOADER_API_IMPORT___(___CSS_LOADER_API_NO_SOURCEMAP_IMPORT___);
// Module
___CSS_LOADER_EXPORT___.push([module.id, "/* @import './b.css';  */\n.a {\n  font-size: 16px; }\n\n#test {\n  color: red; }\n", ""]);
// Exports
export default ___CSS_LOADER_EXPORT___;

二、例子

css的打包结果主要有两部分，一部分是css-loader对css进行的模块化导出，另一部分是style-loader对css的处理（添加到head）。

下面有一个打包真实例子，只贴了modules对象部分：

var __webpack_modules__ = ({

"../../node_modules/css-loader/dist/cjs.js!../../node_modules/sass-loader/dist/cjs.js!./css/a.scss":
/*!*********************************************************************************************************!*\
  !*** ../../node_modules/css-loader/dist/cjs.js!../../node_modules/sass-loader/dist/cjs.js!./css/a.scss ***!
  \*********************************************************************************************************/
((module, __webpack_exports__, __webpack_require__) => {

__webpack_require__.r(__webpack_exports__);
__webpack_require__.d(__webpack_exports__, {
  "default": () => (__WEBPACK_DEFAULT_EXPORT__)
});
var _node_modules_css_loader_dist_runtime_noSourceMaps_js__WEBPACK_IMPORTED_MODULE_0__ = __webpack_require__(/*! ../../../node_modules/css-loader/dist/runtime/noSourceMaps.js */ "../../node_modules/css-loader/dist/runtime/noSourceMaps.js");
var _node_modules_css_loader_dist_runtime_noSourceMaps_js__WEBPACK_IMPORTED_MODULE_0___default = /*#__PURE__*/__webpack_require__.n(_node_modules_css_loader_dist_runtime_noSourceMaps_js__WEBPACK_IMPORTED_MODULE_0__);
var _node_modules_css_loader_dist_runtime_api_js__WEBPACK_IMPORTED_MODULE_1__ = __webpack_require__(/*! ../../../node_modules/css-loader/dist/runtime/api.js */ "../../node_modules/css-loader/dist/runtime/api.js");
var _node_modules_css_loader_dist_runtime_api_js__WEBPACK_IMPORTED_MODULE_1___default = /*#__PURE__*/__webpack_require__.n(_node_modules_css_loader_dist_runtime_api_js__WEBPACK_IMPORTED_MODULE_1__);
// Imports
var ___CSS_LOADER_EXPORT___ = _node_modules_css_loader_dist_runtime_api_js__WEBPACK_IMPORTED_MODULE_1___default()((_node_modules_css_loader_dist_runtime_noSourceMaps_js__WEBPACK_IMPORTED_MODULE_0___default()));
// Module
___CSS_LOADER_EXPORT___.push([module.id, "/* @import './b.css';  */\n.a {\n  font-size: 16px; }\n\n#test {\n  color: red; }\n", ""]);
// Exports
const __WEBPACK_DEFAULT_EXPORT__ = (___CSS_LOADER_EXPORT___);
}),

"../../node_modules/css-loader/dist/runtime/api.js":
/*!*********************************************************!*\
  !*** ../../node_modules/css-loader/dist/runtime/api.js ***!
  \*********************************************************/
((module) => {
/*
  MIT License http://www.opensource.org/licenses/mit-license.php
  Author Tobias Koppers @sokra
*/
module.exports = function (cssWithMappingToString) {
  var list = []; // return the list of modules as css string

  list.toString = function toString() {
    return this.map(function (item) {
      // ...
    }).join("");
  }; // import a list of modules into the list

  list.i = function i(modules, media, dedupe, supports, layer) {
    // ...
  };
  return list;
};
}),

"../../node_modules/css-loader/dist/runtime/noSourceMaps.js":
/*!******************************************************************!*\
  !*** ../../node_modules/css-loader/dist/runtime/noSourceMaps.js ***!
  \******************************************************************/
((module) => {
module.exports = function (i) {
  return i[1];
};
}),

"./css/a.scss":
/*!********************!*\
  !*** ./css/a.scss ***!
  \********************/
((__unused_webpack_module, __webpack_exports__, __webpack_require__) => {

__webpack_require__.r(__webpack_exports__);
__webpack_require__.d(__webpack_exports__, {
  "default": () => (__WEBPACK_DEFAULT_EXPORT__)
});
var _node_modules_style_loader_dist_runtime_injectStylesIntoStyleTag_js__WEBPACK_IMPORTED_MODULE_0__ = __webpack_require__(/*! !../../../node_modules/style-loader/dist/runtime/injectStylesIntoStyleTag.js */ "../../node_modules/style-loader/dist/runtime/injectStylesIntoStyleTag.js");
var _node_modules_style_loader_dist_runtime_injectStylesIntoStyleTag_js__WEBPACK_IMPORTED_MODULE_0___default = /*#__PURE__*/__webpack_require__.n(_node_modules_style_loader_dist_runtime_injectStylesIntoStyleTag_js__WEBPACK_IMPORTED_MODULE_0__);
var _node_modules_style_loader_dist_runtime_styleDomAPI_js__WEBPACK_IMPORTED_MODULE_1__ = __webpack_require__(/*! !../../../node_modules/style-loader/dist/runtime/styleDomAPI.js */ "../../node_modules/style-loader/dist/runtime/styleDomAPI.js");
var _node_modules_style_loader_dist_runtime_styleDomAPI_js__WEBPACK_IMPORTED_MODULE_1___default = /*#__PURE__*/__webpack_require__.n(_node_modules_style_loader_dist_runtime_styleDomAPI_js__WEBPACK_IMPORTED_MODULE_1__);
var _node_modules_style_loader_dist_runtime_insertBySelector_js__WEBPACK_IMPORTED_MODULE_2__ = __webpack_require__(/*! !../../../node_modules/style-loader/dist/runtime/insertBySelector.js */ "../../node_modules/style-loader/dist/runtime/insertBySelector.js");
var _node_modules_style_loader_dist_runtime_insertBySelector_js__WEBPACK_IMPORTED_MODULE_2___default = /*#__PURE__*/__webpack_require__.n(_node_modules_style_loader_dist_runtime_insertBySelector_js__WEBPACK_IMPORTED_MODULE_2__);
var _node_modules_style_loader_dist_runtime_setAttributesWithoutAttributes_js__WEBPACK_IMPORTED_MODULE_3__ = __webpack_require__(/*! !../../../node_modules/style-loader/dist/runtime/setAttributesWithoutAttributes.js */ "../../node_modules/style-loader/dist/runtime/setAttributesWithoutAttributes.js");
var _node_modules_style_loader_dist_runtime_setAttributesWithoutAttributes_js__WEBPACK_IMPORTED_MODULE_3___default = /*#__PURE__*/__webpack_require__.n(_node_modules_style_loader_dist_runtime_setAttributesWithoutAttributes_js__WEBPACK_IMPORTED_MODULE_3__);
var _node_modules_style_loader_dist_runtime_insertStyleElement_js__WEBPACK_IMPORTED_MODULE_4__ = __webpack_require__(/*! !../../../node_modules/style-loader/dist/runtime/insertStyleElement.js */ "../../node_modules/style-loader/dist/runtime/insertStyleElement.js");
var _node_modules_style_loader_dist_runtime_insertStyleElement_js__WEBPACK_IMPORTED_MODULE_4___default = /*#__PURE__*/__webpack_require__.n(_node_modules_style_loader_dist_runtime_insertStyleElement_js__WEBPACK_IMPORTED_MODULE_4__);
var _node_modules_style_loader_dist_runtime_styleTagTransform_js__WEBPACK_IMPORTED_MODULE_5__ = __webpack_require__(/*! !../../../node_modules/style-loader/dist/runtime/styleTagTransform.js */ "../../node_modules/style-loader/dist/runtime/styleTagTransform.js");
var _node_modules_style_loader_dist_runtime_styleTagTransform_js__WEBPACK_IMPORTED_MODULE_5___default = /*#__PURE__*/__webpack_require__.n(_node_modules_style_loader_dist_runtime_styleTagTransform_js__WEBPACK_IMPORTED_MODULE_5__);
var _node_modules_css_loader_dist_cjs_js_node_modules_sass_loader_dist_cjs_js_a_scss__WEBPACK_IMPORTED_MODULE_6__ = __webpack_require__(/*! !!../../../node_modules/css-loader/dist/cjs.js!../../../node_modules/sass-loader/dist/cjs.js!./a.scss */ "../../node_modules/css-loader/dist/cjs.js!../../node_modules/sass-loader/dist/cjs.js!./css/a.scss");

var options = {};

options.styleTagTransform = (_node_modules_style_loader_dist_runtime_styleTagTransform_js__WEBPACK_IMPORTED_MODULE_5___default());
options.setAttributes = (_node_modules_style_loader_dist_runtime_setAttributesWithoutAttributes_js__WEBPACK_IMPORTED_MODULE_3___default());

options.insert = _node_modules_style_loader_dist_runtime_insertBySelector_js__WEBPACK_IMPORTED_MODULE_2___default().bind(null, "head");
    
options.domAPI = (_node_modules_style_loader_dist_runtime_styleDomAPI_js__WEBPACK_IMPORTED_MODULE_1___default());
options.insertStyleElement = (_node_modules_style_loader_dist_runtime_insertStyleElement_js__WEBPACK_IMPORTED_MODULE_4___default());

var update = _node_modules_style_loader_dist_runtime_injectStylesIntoStyleTag_js__WEBPACK_IMPORTED_MODULE_0___default()(_node_modules_css_loader_dist_cjs_js_node_modules_sass_loader_dist_cjs_js_a_scss__WEBPACK_IMPORTED_MODULE_6__["default"], options);

const __WEBPACK_DEFAULT_EXPORT__ = (_node_modules_css_loader_dist_cjs_js_node_modules_sass_loader_dist_cjs_js_a_scss__WEBPACK_IMPORTED_MODULE_6__["default"] && _node_modules_css_loader_dist_cjs_js_node_modules_sass_loader_dist_cjs_js_a_scss__WEBPACK_IMPORTED_MODULE_6__["default"].locals ? _node_modules_css_loader_dist_cjs_js_node_modules_sass_loader_dist_cjs_js_a_scss__WEBPACK_IMPORTED_MODULE_6__["default"].locals : undefined);
}),

"../../node_modules/style-loader/dist/runtime/injectStylesIntoStyleTag.js":
/*!********************************************************************************!*\
  !*** ../../node_modules/style-loader/dist/runtime/injectStylesIntoStyleTag.js ***!
  \********************************************************************************/
((module) => {
var stylesInDOM = [];
function getIndexByIdentifier(identifier) {
  var result = -1;

  for (var i = 0; i < stylesInDOM.length; i++) {
    if (stylesInDOM[i].identifier === identifier) {
      result = i;
      break;
    }
  }

  return result;
}

function modulesToDom(list, options) {
  var idCountMap = {};
  var identifiers = [];

  for (var i = 0; i < list.length; i++) {
    var item = list[i];
    var id = options.base ? item[0] + options.base : item[0];
    var count = idCountMap[id] || 0;
    var identifier = "".concat(id, " ").concat(count);
    idCountMap[id] = count + 1;
    var indexByIdentifier = getIndexByIdentifier(identifier);
    var obj = {
      css: item[1],
      media: item[2],
      sourceMap: item[3],
      supports: item[4],
      layer: item[5]
    };

    if (indexByIdentifier !== -1) {
      stylesInDOM[indexByIdentifier].references++;
      stylesInDOM[indexByIdentifier].updater(obj);
    } else {
      var updater = addElementStyle(obj, options);
      options.byIndex = i;
      stylesInDOM.splice(i, 0, {
        identifier: identifier,
        updater: updater,
        references: 1
      });
    }

    identifiers.push(identifier);
  }

  return identifiers;
}

function addElementStyle(obj, options) {
  var api = options.domAPI(options);
  api.update(obj);

  var updater = function updater(newObj) {
    if (newObj) {
      if (newObj.css === obj.css && newObj.media === obj.media && newObj.sourceMap === obj.sourceMap && newObj.supports === obj.supports && newObj.layer === obj.layer) {
        return;
      }

      api.update(obj = newObj);
    } else {
      api.remove();
    }
  };

  return updater;
}

module.exports = function (list, options) {
  options = options || {};
  list = list || [];
  var lastIdentifiers = modulesToDom(list, options);
  return function update(newList) {
    newList = newList || [];

    for (var i = 0; i < lastIdentifiers.length; i++) {
      var identifier = lastIdentifiers[i];
      var index = getIndexByIdentifier(identifier);
      stylesInDOM[index].references--;
    }

    var newLastIdentifiers = modulesToDom(newList, options);

    for (var _i = 0; _i < lastIdentifiers.length; _i++) {
      var _identifier = lastIdentifiers[_i];

      var _index = getIndexByIdentifier(_identifier);

      if (stylesInDOM[_index].references === 0) {
        stylesInDOM[_index].updater();

        stylesInDOM.splice(_index, 1);
      }
    }

    lastIdentifiers = newLastIdentifiers;
  };
};

}),

"../../node_modules/style-loader/dist/runtime/insertBySelector.js":
/*!************************************************************************!*\
  !*** ../../node_modules/style-loader/dist/runtime/insertBySelector.js ***!
  \************************************************************************/
((module) => {
var memo = {};
/* istanbul ignore next  */

function getTarget(target) {
  if (typeof memo[target] === "undefined") {
    var styleTarget = document.querySelector(target); // Special case to return head of iframe instead of iframe itself

    if (window.HTMLIFrameElement && styleTarget instanceof window.HTMLIFrameElement) {
      try {
        // This will throw an exception if access to iframe is blocked
        // due to cross-origin restrictions
        styleTarget = styleTarget.contentDocument.head;
      } catch (e) {
        // istanbul ignore next
        styleTarget = null;
      }
    }

    memo[target] = styleTarget;
  }

  return memo[target];
}
/* istanbul ignore next  */

function insertBySelector(insert, style) {
  var target = getTarget(insert);

  if (!target) {
    throw new Error("Couldn't find a style target. This probably means that the value for the 'insert' parameter is invalid.");
  }

  target.appendChild(style);
}

module.exports = insertBySelector;

}),

"../../node_modules/style-loader/dist/runtime/insertStyleElement.js":
/*!**************************************************************************!*\
  !*** ../../node_modules/style-loader/dist/runtime/insertStyleElement.js ***!
  \**************************************************************************/
((module) => {
/* istanbul ignore next  */
function insertStyleElement(options) {
  var element = document.createElement("style");
  options.setAttributes(element, options.attributes);
  options.insert(element, options.options);
  return element;
}

module.exports = insertStyleElement;
}),

"../../node_modules/style-loader/dist/runtime/setAttributesWithoutAttributes.js":
/*!**************************************************************************************!*\
  !*** ../../node_modules/style-loader/dist/runtime/setAttributesWithoutAttributes.js ***!
  \**************************************************************************************/
((module, __unused_webpack_exports, __webpack_require__) => {
/* istanbul ignore next  */
function setAttributesWithoutAttributes(styleElement) {
  var nonce =  true ? __webpack_require__.nc : 0;
  if (nonce) {
    styleElement.setAttribute("nonce", nonce);
  }
}
module.exports = setAttributesWithoutAttributes;
}),

"../../node_modules/style-loader/dist/runtime/styleDomAPI.js":
/*!*******************************************************************!*\
  !*** ../../node_modules/style-loader/dist/runtime/styleDomAPI.js ***!
  \*******************************************************************/
((module) => {
/* istanbul ignore next  */
function apply(styleElement, options, obj) {
  var css = "";

  if (obj.supports) {
    css += "@supports (".concat(obj.supports, ") {");
  }

  if (obj.media) {
    css += "@media ".concat(obj.media, " {");
  }

  var needLayer = typeof obj.layer !== "undefined";

  if (needLayer) {
    css += "@layer".concat(obj.layer.length > 0 ? " ".concat(obj.layer) : "", " {");
  }

  css += obj.css;

  if (needLayer) {
    css += "}";
  }

  if (obj.media) {
    css += "}";
  }

  if (obj.supports) {
    css += "}";
  }

  var sourceMap = obj.sourceMap;

  if (sourceMap && typeof btoa !== "undefined") {
    css += "\n/*# sourceMappingURL=data:application/json;base64,".concat(btoa(unescape(encodeURIComponent(JSON.stringify(sourceMap)))), " */");
  } // For old IE

  /* istanbul ignore if  */
  options.styleTagTransform(css, styleElement, options.options);
}

function removeStyleElement(styleElement) {
  // istanbul ignore if
  if (styleElement.parentNode === null) {
    return false;
  }

  styleElement.parentNode.removeChild(styleElement);
}
/* istanbul ignore next  */
function domAPI(options) {
  var styleElement = options.insertStyleElement(options);
  return {
    update: function update(obj) {
      apply(styleElement, options, obj);
    },
    remove: function remove() {
      removeStyleElement(styleElement);
    }
  };
}
module.exports = domAPI;
}),

"../../node_modules/style-loader/dist/runtime/styleTagTransform.js":
/*!*************************************************************************!*\
  !*** ../../node_modules/style-loader/dist/runtime/styleTagTransform.js ***!
  \*************************************************************************/
((module) => {
/* istanbul ignore next  */
function styleTagTransform(css, styleElement) {
  if (styleElement.styleSheet) {
    styleElement.styleSheet.cssText = css;
  } else {
    while (styleElement.firstChild) {
      styleElement.removeChild(styleElement.firstChild);
    }

    styleElement.appendChild(document.createTextNode(css));
  }
}
module.exports = styleTagTransform;
}),

"./c.js":
/*!**************!*\
  !*** ./c.js ***!
  \**************/
((__unused_webpack_module, __webpack_exports__, __webpack_require__) => {

__webpack_require__.r(__webpack_exports__);
__webpack_require__.d(__webpack_exports__, {
  "default": () => (__WEBPACK_DEFAULT_EXPORT__)
});
const c = 1
const __WEBPACK_DEFAULT_EXPORT__ = (c);
})
/******/ 	});