How to write a simple webpack loader?

Question

webpack

Hello, trying to figure out the device of loaders, I'm wondering how the parameters for the loader are passed?
For example, a simple loader https://github.com/mortenson/emit-file-loader#readme
whose code

const loaderUtils = require('loader-utils');
module.exports = function loader(content, map, meta) {
  const options = loaderUtils.getOptions(this);
  if (options.output) {
    const name = loaderUtils.interpolateName(this, options.output, options);
    this.emitFile(name, content);
  }
  this.callback(null, content, map, meta);
};

You can use it

require(`emit-file-loader?{output:"somefile.css"}!../src/someFileExample.txt`)

Where do I pass parameters to the loader output:"somefile.css"and it receives an argument from execution ../src/someFileExample.txt
How to pass an argument and parameters using a function inside js, for example

require("emit-file-loader")({output:"somefile.css})("someTextToFileHere");

That is, I need to pass the value toloaderUtils.getOptions(this)

Answer 1

The first thing to understand here is that the loader works at compile-time , and your js code where require - at run-time .
The loader receives the source text of the file (or the text issued by the previous loader) as input and outputs its own text. The last loader (it is the first in the webpack loader chain) should produce valid js.
Of course, your loader can generate a function in the output file, but it will not be able to affect the compilation process much.