Q:

Gulp write file if none exist

How can I make gulp write a file only if there is no existing file.
The bellow solution works for gulp 4.0 which is in alpha.
// for when gulp 4.0 releases
.pipe(gulp.dest(conf.plugScss.dist, {overwrite: false}))

A:

There is no exact equivalent in gulp 3.x, but you can use gulp-changed to achieve the same thing. 
gulp-changed is usually used to write only those files that have changed since the last time they were written to the destination folder. However you can provide a custom hasChanged function. In your case you can write a function does nothing but check if the file already exists using fs.stat():
var gulp = require('gulp');
var changed = require('gulp-changed');
var fs = require('fs');

function compareExistence(stream, cb, sourceFile, targetPath) {
  fs.stat(targetPath, function(err, stats) {
    if (err) {
       stream.push(sourceFile);
    }
    cb();
  });
}

gulp.task('default', function() {
  return gulp.src(/*...*/)
    .pipe(changed(conf.plugScss.dist, {hasChanged: compareExistence}))
    .pipe(gulp.dest(conf.plugScss.dist));
});

