NumPy binary file parser for javascript

loadnpy.js

// Client-side parser for .npy files
// See the specification: http://docs.scipy.org/doc/numpy-dev/neps/npy-format.html
var NumpyLoader = (function () {
    function asciiDecode(buf) {
        return String.fromCharCode.apply(null, new Uint8Array(buf));
    }

    function readUint16LE(buffer) {
        var view = new DataView(buffer);
        var val = view.getUint8(0);
        val |= view.getUint8(1) << 8;
        return val;
    }

    function fromArrayBuffer(buf) {
      // Check the magic number
      var magic = asciiDecode(buf.slice(0,6));
      if (magic.slice(1,6) != 'NUMPY') {
          throw new Error('unknown file type');
      }

      var version = new Uint8Array(buf.slice(6,8)),
          headerLength = readUint16LE(buf.slice(8,10)),
          headerStr = asciiDecode(buf.slice(10, 10+headerLength));
          offsetBytes = 10 + headerLength;
          //rest = buf.slice(10+headerLength);  XXX -- This makes a copy!!! https://www.khronos.org/registry/typedarray/specs/latest/#5

      // Hacky conversion of dict literal string to JS Object
      eval("var info = " + headerStr.toLowerCase().replace('(','[').replace('),',']'));
    
      // Intepret the bytes according to the specified dtype
      var data;
      if (info.descr === "|u1") {
          data = new Uint8Array(buf, offsetBytes);
      } else if (info.descr === "|i1") {
          data = new Int8Array(buf, offsetBytes);
      } else if (info.descr === "<u2") {
          data = new Uint16Array(buf, offsetBytes);
      } else if (info.descr === "<i2") {
          data = new Int16Array(buf, offsetBytes);
      } else if (info.descr === "<u4") {
          data = new Uint32Array(buf, offsetBytes);
      } else if (info.descr === "<i4") {
          data = new Int32Array(buf, offsetBytes);
      } else if (info.descr === "<f4") {
          data = new Float32Array(buf, offsetBytes);
      } else if (info.descr === "<f8") {
          data = new Float64Array(buf, offsetBytes);
      } else {
          throw new Error('unknown numeric dtype')
      }

      return {
          shape: info.shape,
          fortran_order: info.fortran_order,
          data: data
      };
    }

    function open(file, callback) {
        var reader = new FileReader();
        reader.onload = function() {
            // the file contents have been read as an array buffer
            var buf = reader.result;
            var ndarray = fromArrayBuffer(buf);
            callback(ndarray);
        };
        reader.readAsArrayBuffer(file);
    }

    function ajax(url, callback) {
        var xhr = new XMLHttpRequest();
        xhr.onload = function(e) {
            var buf = xhr.response; // not responseText
            var ndarray = fromArrayBuffer(buf);
            callback(ndarray);
        };
        xhr.open("GET", url, true);
        xhr.responseType = "arraybuffer";
        xhr.send(null);
    }

    return {
        open: open,
        ajax: ajax
    };
})();