index.js

const through = require('through2');
const read = require('read-file');
const PluginError = require('plugin-error');

const PLUGIN_NAME = 'gulp-append-prepend';

function arrayify(obj) {
    return Array.isArray(obj) ? obj : [obj];
}

function filesGetContents(filepaths) {
    filepaths = arrayify(filepaths);

    const filesContents = [];
    for (var i = 0; i < filepaths.length; i++) {
        filesContents.push(read.sync(filepaths[i], 'utf8'));
    }
    return filesContents;
}

function insert(texts, separator, type) {
    if (!texts) {
        throw new PluginError(PLUGIN_NAME, 'Missing text or path !');
    }

    texts = arrayify(texts);

    if (type !== "append" && type !== "prepend") {
        throw new PluginError(PLUGIN_NAME, 'Missing type !');
    }

    if (!separator && separator !== '') {
        separator = "\n";
    }

    const buffers = [];
    for (var i = 0; i < texts.length; i++) {
        if (type === "prepend") {
            buffers.push(Buffer.from(texts[i].trim() + separator));
        } else if (type === "append") {
            buffers.push(Buffer.from(separator + texts[i].trim()));
        }
    }

    const stream = through.obj(function (file, enc, cb) {
        if (file.isStream()) {
            this.emit('error', new PluginError(PLUGIN_NAME, 'Streams are not supported !'));
            return cb();
        }

        if (file.isBuffer()) {
            const concat = [];
            if (type === "append") {
                concat.push(file.contents);
            }
            for (var i = 0; i < buffers.length; i++) {
                concat.push(buffers[i]);
            }
            if (type === "prepend") {
                concat.push(file.contents);
            }

            file.contents = Buffer.concat(concat);
        }

        this.push(file);
        cb();
    });

    return stream;
}

module.exports.appendFile = function (filepath, separator) {
    return insert(filesGetContents(filepath), separator, "append");
};

module.exports.prependFile = function (filepath, separator) {
    return insert(filesGetContents(filepath), separator, "prepend");
};

module.exports.appendText = function (text, separator) {
    return insert(text, separator, "append");
};

module.exports.prependText = function (text, separator) {
    return insert(text, separator, "prepend");
};