var mapSources = require('@gulp-sourcemaps/map-sources');

gulp.src(...)

.pipe(sourcemaps.init())

.pipe(mapSources(function(sourcePath, file) {

return '../' + sourcePath;

}))

.pipe(sourcemaps.write())

.pipe(gulp.dest(...))

Takes a map function as the only argument. Returns an `objectMode` Transform stream.

The map function is called once per value of the `sources` array of a `sourceMap` attached to each [`Vinyl`][vinyl-url] object passed through the stream. The map function is called with the `sourcePath` string from the `sources` array and the `file` object it originated from. The return value replaces the original value in the array.

If a `Vinyl` object doesn't have a `sourceMap` or `sourceMap.sources` property, the file is passed through the stream without having the `mapFn` called.

All `sources` are normalized to use `/` instead of `\\` as path separators.

[vinyl-url]: https://github.com/gulpjs/vinyl