array-buffer-byte-length 
Get the byte length of an ArrayBuffer, even in engines without a .byteLength method.
Example
const assert = require('assert');
const byteLength = require('array-buffer-byte-length');
assert.equal(byteLength([]), NaN, 'an array is not an ArrayBuffer, yields NaN');
assert.equal(byteLength(new ArrayBuffer(0)), 0, 'ArrayBuffer of byteLength 0, yields 0');
Tests
Simply clone the repo, npm install, and run npm test