Streaming CSV parser using the Web Streams API

CsvDecodeStream.ts

/* eslint-disable @typescript-eslint/no-non-null-assertion */

class _CsvDecodeStream implements Transformer<any, Record<string, string | number>[]> {
    line: (string | number)[] = [];

    buffer = "";
    isQuoted = false;
    wasQuote = false;

    constructor(private withHeaders = true) {}
    headers: typeof this.line | null = null;

    sendLine(controller: TransformStreamDefaultController) {
        if (!this.withHeaders) {
            controller.enqueue(this.line);
        } else {
            if (this.headers === null) {
                this.headers = this.line;
            } else {
                controller.enqueue(Object.fromEntries(this.line.map((v, i) => [this.headers![i] || i, v])));
            }
        }
        this.line = [];
    }

    addBuffer() {
        const t = this.buffer.trim();
        const p = parseFloat(t);
        if (!isNaN(p) && t === p.toString())
            this.line.push(p);
        else 
            this.line.push(t);
        this.buffer = "";
    }

    private parseChar(char: string, controller: TransformStreamDefaultController) {
        // if we see a quote, we need to check if it's a quote or a double quote escape
        // first quote 
        if (this.isQuoted && char === '"' && !this.wasQuote) {
            this.wasQuote = true;
            return;
        // second quote
        } else if (this.isQuoted && char === '"' && this.wasQuote) {
            this.buffer += '"';
            this.wasQuote = false;
            return;
        }
        // enter/leave quoted mode 
        if (char === '"') {
            this.isQuoted = !this.isQuoted;
            return;
        } else if (this.isQuoted && this.wasQuote) {
            this.isQuoted = false;
        }
        this.wasQuote = false;
        // if we're not in quoted mode, we need to check if we're at the end of a field
        if (!this.isQuoted && char === ',') {
            this.addBuffer();
            return;
        }
        // if we're not in quoted mode and we're at the end of a line
        if (!this.isQuoted && char === '\n') {
            this.addBuffer();
            this.sendLine(controller);
            return;
        }
        this.buffer += char;

    }

    td = new TextDecoder();
    transform(chunk: any, controller: TransformStreamDefaultController) {
        let data;
        if (typeof chunk == "string") data = chunk;
        else if (typeof Buffer !== 'undefined' && chunk instanceof Buffer) data = chunk.toString();
        else if (chunk instanceof ArrayBuffer) data = this.td.decode(chunk, {stream:true});
        else if (chunk instanceof Uint8Array) data = this.td.decode(chunk, {stream:true});
        else throw new Error("Unsupported chunk type: " + typeof chunk);

        for (let i = 0; i < data.length; i++)
            this.parseChar(data[i], controller);
    }
    flush(controller: TransformStreamDefaultController) {
        if (this.buffer.length > 0) 
            this.line.push(this.buffer);
        if (this.line.length > 0) 
            this.sendLine(controller);
    }

}

export function CsvDecodeStream(withHeaders = true) {
    return new TransformStream(new _CsvDecodeStream(withHeaders));
}