Like Node.js 'readline' for HTML5 File API.

line-reader.js

const DEFAULT_CHUNK_SIZE = 1024;

class LineReader {
  constructor(options) {
    this.chunkSize = options.chunkSize || DEFAULT_CHUNK_SIZE;

    this.onLine = options.onLine;
    this.onEnd = options.onEnd;

    this.reader = new FileReader();

    this.canRead = true;
    this.chunk = '';
    this.lines = null;

    this.reader.addEventListener('load', this.handleReaderLoad.bind(this));
  }

  handleReaderLoad() {
    this.chunk += this.reader.result;

    const hasNewLine = /\n|\r/.test(this.chunk);

    if (hasNewLine) {
      this.lines = this.chunk.match(/[^\r\n]+/g);

      if (this.hasMoreData()) {
        const lineFinished = this.chunk[this.chunk.length - 1] === '\n';

        this.chunk = lineFinished ? '' : this.lines.pop();
      }

      this.step();
      return;
    }

    if (this.hasMoreData()) {
      this.read();
      return;
    }

    if (this.chunk.length > 0) {
      this.onLine(this.chunk);
      this.onEnd();
      return;
    }

    this.onEnd();
  }

  hasMoreData() {
    return this.readPos < this.fileLength;
  }

  step() {
    if (this.lines.length === 0) {
      if (this.hasMoreData()) {
        this.read();
        return;
      }

      this.onEnd();
      return;
    }

    if (this.canRead) {
      this.onLine(this.lines.shift());
      this.step();
      return;
    }

    this.onEnd();
  }

  read(file) {
    if (file) {
      this.file = file;
      this.fileLength = file.size;
      this.readPos = 0;
      this.chunk = '';
      this.lines = [];
    }

    this.readPos += this.chunkSize;

    this.reader.readAsText(
      this.file.slice(this.readPos, this.readPos + this.chunkSize)
    );
  }

  abort() {
    this.canRead = false;
  }
}

window.LineReader = LineReader;