Get host machine endianness using JavaScript Typed Arrays (polyfill for `os.endianness()` in node.js)

endianness.js

function endianness () {
  var b = new ArrayBuffer(4);
  var a = new Uint32Array(b);
  var c = new Uint8Array(b);
  a[0] = 0xdeadbeef;
  if (c[0] == 0xef) return 'LE';
  if (c[0] == 0xde) return 'BE';
  throw new Error('unknown endianness');
}

endianness();
// "LE"

One line version:

var isBigEndian = new Uint8Array(new Uint32Array([0x12345678]).buffer)[0] === 0x12;

var isLittleEndian = new Uint8Array(new Uint32Array([0x12345678]).buffer)[0] === 0x78;

Be careful with this one-line code - you can't blindly trust TypedArray.buffer property: see notes on https://nodejs.org/api/buffer.html#buffer_buf_buffer and https://nodejs.org/api/buffer.html#buffer_buf_byteoffset