multi-write.js

var destinations = new WeakMap()

function MultiWrite(writable) {
    var destinations = [writable]

    var stream = new BaseWritableStream({
        write: function (chunk) {
            return Promise.all(destinations.map(function (dest) {
                return dest.write(chunk)
            }))
        },
        close: function () {
            return Promise.all(destinations.map(function (dest) {
                return dest.close()
            }))
        },
        dispose: function (reason) {
            return Promise.all(destinations.map(function (dest) {
                return dest.dispose(reason)
            }))
        }
    })

    stream.addWritable = function (stream) {
        destinations.push(stream)
    }

    return stream
}

function pipe(source, dest, options) {
    if (!options) {
        options = {}
    }

    // multi pipe pipes to a mutable MultiWrite stream
    var targetDest = destinations.get(source)
    if (!targetDest) {
        targetDest = MultiWrite(dest)
        destinations.set(source, targetDest)
    } else {
        targetDest.addWritable(dest)
        return dest
    }

    var close = "close" in options ? Boolean(options.close) : true

    pump()
    return dest

    function pump() {
        if (targetDest.writableState === "writable") {
            moveChunk()
        } else if (targetDest.writableState === "waiting") {
            targetDest.waitForWritable().then(pump)
        } else if (targetDest.writableState === "errored") {
            source.abort()
        } else if (targetDest.writableState === "closed") {
            // NO OP. Terminate pipe(). Do not close source
        }
    }

    function moveChunk() {
        if (source.readableState === "readable") {
            var chunk = source.read()
            targetDest.write(chunk)
            pump()
        } else if (source.readableState === "waiting") {
            source.waitForReadable().then(pump)
        } else if (source.readableState === "finished") {
            if (close) {
                targetDest.close()
            }
        } else if (source.readableState === "errored") {
            targetDest.dispose()
        }
    }
}