Here's what I'm using.

Code

import type { Writable } from 'node:stream'
import type ExcelJS from 'exceljs'

type ReadableStreamResult<T> = {
    readable: ReadableStream<T>
    done: Promise<void>
}

export function workbookToStream(workbook: ExcelJS.Workbook): ReadableStreamResult<Uint8Array> {
    const transform = new TransformStream<Uint8Array, Uint8Array>()
    const done = workbook.xlsx.write(new NodeJsWritableLike<Uint8Array>(transform.writable) as unknown as Writable)

    return {
        readable: transform.readable,
        done,
    }
}

class NodeJsWritableLike<T> {
    writer: WritableStreamDefaultWriter<T>

    constructor(writable: WritableStream<T>) {
        this.writer = writable.getWriter()
    }

    async write(chunk: T, cb: () => void): Promise<void> {
        await this.writer.ready
        await this.writer.write(chunk)
        cb()
    }
    async end(): Promise<void> {
        await this.writer.ready
        await this.writer.close()
    }
}

Explanation

NodeJsWritableLike is a minimal wrapper around WritableStream that implements only the specific methods and signatures of node:stream's Writable that are called internally by workbook.xlsx.write.

stream is passed as an argument to zip.pipe:

exceljs/lib/xlsx/xlsx.js

Lines 720 to 724 in 5bed18b

    
           async write(stream, options) { 
        
             options = options || {}; 
        
             const {model} = this.workbook; 
        
             const zip = new ZipStream.ZipWriter(options.zip); 
        
             zip.pipe(stream);

It's passed as destination to ZipWriter's this.stream:

exceljs/lib/utils/zip-stream.js

Lines 66 to 68 in 5bed18b

    
           pipe(destination, options) { 
        
             return this.stream.pipe(destination, options); 
        
           }

ZipWriter's this.stream is a StreamBuf:

exceljs/lib/utils/zip-stream.js

Line 22 in 5bed18b

this.stream = new StreamBuf();

Upon calling StreamBuf's pipe method, it's added to StreamBuf's this.pipes:

exceljs/lib/utils/stream-buf.js

Lines 342 to 344 in 5bed18b

    
           pipe(destination) { 
        
             // add destination to pipe list & write current buffer 
        
             this.pipes.push(destination);

this.pipes are only iterated in two places, the first to write with a callback:

exceljs/lib/utils/stream-buf.js

Lines 202 to 211 in 5bed18b

    
           async _pipe(chunk) { 
        
             const write = function(pipe) { 
        
               return new Promise(resolve => { 
        
                 pipe.write(chunk.toBuffer(), () => { 
        
                   resolve(); 
        
                 }); 
        
               }); 
        
             }; 
        
             await Promise.all(this.pipes.map(write)); 
        
           },

...and the second to end with no callback:

exceljs/lib/utils/stream-buf.js

Lines 289 to 291 in 5bed18b

    
           this.pipes.forEach(pipe => { 
        
             pipe.end(); 
        
           });

Seems to work well in both browser (Chrome) and server-side (Deno).

Assuming my logic is sound, it should be foolproof, but not necessarily future-proof as the ExcelJS's implementation details could change. I might submit a PR to try and get this properly supported and tested, but for now the wrapper-based method should be good to use as of v4.4.0.

	async write(stream, options) {
	options = options \|\| {};
	const {model} = this.workbook;
	const zip = new ZipStream.ZipWriter(options.zip);
	zip.pipe(stream);

	pipe(destination, options) {
	return this.stream.pipe(destination, options);
	}

	pipe(destination) {
	// add destination to pipe list & write current buffer
	this.pipes.push(destination);

	async _pipe(chunk) {
	const write = function(pipe) {
	return new Promise(resolve => {
	pipe.write(chunk.toBuffer(), () => {
	resolve();
	});
	});
	};
	await Promise.all(this.pipes.map(write));
	},

Uh oh!

[Q] What's the status to support Web Stream ? #2753

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