nodejs streams-将错误转换为默认值

./lib/create-fs-with-default.js

module.exports = // or export default  if you use es6 modules
    function(filepath, def = "Could not read file") {
        // We open the file normally
        const _in = fs.createReadStream(filepath);
        // We'll need a list of targets later on
        let _piped = [];
        // Here's a handler that end's all piped outputs with the default value.
        const _handler = (e) => {
            if (!_piped.length) {
                throw e;
            }

            _piped.forEach(
                out => out.end(def)
            );
        };
        _in.once("error", _handler);

        // We keep the original `pipe` method in a variable
        const _orgPipe = _in.pipe;
        // And override it with our alternative version...
        _in.pipe = function(to, ...args) {
            const _out = _orgPipe.call(this, to, ...args);

            // ...which, apart from calling the original, also records the outputs
            _piped.push(_out);
            return _out;
        }
        // Optionally we could handle `unpipe` method here.

        // Here we remove the handler once data flow is started.
        _in.once("data", () => _in.removeListener("error", _handler));
        // And pause the stream again so that `data` listener doesn't consume the first chunk.
        _in.pause();

        // Finally we return the read stream
        return _in;
    };

const createReadStreamWithDefault = require("./lib/create-fs-with-default");
const inputStream = fs.createReadStream(getTrgFilePath(), "Invalid input!");
// ... and at some point
inputStream.pipe(someOutput);