const majo = require('majo')

const stream = majo()

// Given that you have js/app.js js/index.js
stream
  .source('js/**')
  .use(ignoreSomeFiles)
  .dest('dist')
  .then(() => {
    // Now you got filtered files
  })

function ignoreSomeFiles(stream) {
  for (const filename in stream.files) {
    const content = stream.fileContents(filename)
    // Remove it if content has specific string
    if (/some-string/.test(content)) {
      delete stream.files[filename]
    }
  }
}