AMR/node_modules/vue-loader/lib/template-compiler/modules/transform-srcset.js

// vue compiler module for transforming `img:srcset` to a number of `require`s

const urlToRequire = require('../url-to-require')

module.exports = () => ({
  postTransformNode: node => {
    transform(node)
  }
})

function transform (node) {
  const tags = ['img', 'source']

  if (tags.indexOf(node.tag) !== -1 && node.attrs) {
    node.attrs.forEach(attr => {
      if (attr.name === 'srcset') {
        // same logic as in transform-require.js
        const value = attr.value
        const isStatic = value.charAt(0) === '"' && value.charAt(value.length - 1) === '"'
        if (!isStatic) {
          return
        }

        // http://w3c.github.io/html/semantics-embedded-content.html#ref-for-image-candidate-string-5
        const escapedSpaceCharacters = /( |\\t|\\n|\\f|\\r)+/g

        const imageCandidates = value.substr(1, value.length - 2).split(',').map(s => {
          // The attribute value arrives here with all whitespace, except normal spaces, represented by escape sequences
          const [url, descriptor] = s.replace(escapedSpaceCharacters, ' ').trim().split(' ', 2)
          return { require: urlToRequire(url), descriptor: descriptor }
        })

        // "require(url1)"
        // "require(url1) 1x"
        // "require(url1), require(url2)"
        // "require(url1), require(url2) 2x"
        // "require(url1) 1x, require(url2)"
        // "require(url1) 1x, require(url2) 2x"
        const code = imageCandidates.map(
          ({ require, descriptor }) => `${require} + "${descriptor ? ' ' + descriptor : ''}, " + `
        ).join('').slice(0, -6).concat('"').replace(/ \+ ""$/, '')

        attr.value = code
      }
    })
  }
}