What does this mean: TypeError: dest.on is not a function

Question

I am a newbie, following a tutorial about modern webdev workflow. I get this error message in my console. TypeError: dest.on is not a function I know, there are related questions and answers here. But I don't understand them. Because I don't know what "dest.on" is related to and what does it do. This is the code so far:

var gulp = require("gulp");
var sass = require("gulp-sass");
var sourcemaps = require("gulp-sourcemaps");
var autoprefixer = require("auto-prefixer");
var imagemin = require("gulp-imagemin");
var browserSync = require("browser-sync").create();

gulp.task("css", function() {
  return gulp
    .src("src/sass/**/*.scss")
    .pipe(sourcemaps.init())
    .pipe(sass({ outputStyle: "compressed" }).on("error", sass.logError))
    .pipe(
      autoprefixer({
        browsers: ["last 2 versions"]
      })
    )
    .pipe(sourcemaps.write("./maps"))
    .pipe(gulp.dest("dist/css"));
});

Could anyone explain, what the error message means and how I can solve this particular problem? I am sorry for the redundancy but I found no solution in the existing answers.

Edit:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: Maybe it helps adding the package.json

{
  "name": "sitepointresponsivewebsite",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "auto-prefixer": "^0.4.2",
    "browser-sync": "^2.26.3",
    "gulp": "^4.0.0",
    "gulp-imagemin": "^5.0.3",
    "gulp-sass": "^4.0.2",
    "gulp-sourcemaps": "^2.6.5"
  }
}

This is the complete terminal error message

Beratungs-MacBook-Pro-2:sitepointResponsiveWebsite Beratung1$ gulp css
[14:04:02] Using gulpfile ~/Desktop/sitepointResponsiveWebsite/gulpfile.js
[14:04:02] Starting 'css'...
[14:04:02] 'css' errored after 12 ms
[14:04:02] TypeError: dest.on is not a function
    at DestroyableTransform.Readable.pipe (/Users/Beratung1/Desktop/sitepointResponsiveWebsite/node_modules/readable-stream/lib/_stream_readable.js:564:8)
    at /Users/Beratung1/Desktop/sitepointResponsiveWebsite/gulpfile.js:13:6
    at taskWrapper (/Users/Beratung1/Desktop/sitepointResponsiveWebsite/node_modules/undertaker/lib/set-task.js:13:15)
    at bound (domain.js:396:14)
    at runBound (domain.js:409:12)
    at asyncRunner (/Users/Beratung1/Desktop/sitepointResponsiveWebsite/node_modules/async-done/index.js:55:18)
    at process._tickCallback (internal/process/next_tick.js:61:11)

Answer 1

With unfamiliar error messages like this, the stack trace is usually the most helpful.

at DestroyableTransform.Readable.pipe (/Users/Beratung1/Desktop/sitepointResponsiveWebsite/node_modules/readable-stream/lib/_stream_readable.js:564:8)
at /Users/Beratung1/Desktop/sitepointResponsiveWebsite/gulpfile.js:13:6

The error is happening in the pipe() method in readable-stream. dest is the name of the first argument to pipe(); the fact that "dest.on" does not exist means that something you passed to pipe() is not actually a stream.

The second stack frame refers to line 13, which is this pipe() call:

.pipe(
  autoprefixer({
    browsers: ["last 2 versions"]
  })
)

So, the return value of autoprefixer() is not a stream.

autoprefixer is:

var autoprefixer = require("auto-prefixer");

Looking at that module on npm: https://www.npmjs.com/package/auto-prefixer. Looks weird, it definitely isn't a stream API though.

The real autoprefixer module is called autoprefixer, no dash. You can use it together with gulp-postcss as described here: https://www.npmjs.com/package/autoprefixer#gulp

var postcss = require('gulp-postcss')
var autoprefixer = require('autoprefixer')

// .. etc ..
  .pipe(postcss([ autoprefixer({
    // .. options ..
  }) ]))