// Install and import through2
// npm install --save-dev through2
const through = require('through2');

// Declare the function
var myfn = function () {
    return through.obj(function (input, encoding, callback) {
        // Get current stream content.
        let content = String(input.contents);

        // Do some transformations
        // ...

        // Return a new stream with transformed content.
        let output = input.clone();
        output.contents = new Buffer(content);

        // Resume next pipe.
        callback(null, output);
    });
};

// Usage.
gulp.src('...').pipe(myfn()).pipe(gulp.dest('...'));