This is the minimum amount of JavaScript (74 characters) to detect the endianness of a platform.

minimal-endianness.js

// The magic number 0xFEFF is selected for familiarity with the commonly-used
// byte-order mark (BOM) character (https://en.wikipedia.org/wiki/Byte_order_mark).
const isLE = new Uint8Array(new Uint16Array([0xFEFF]).buffer)[0] === 0xFF;

// Golfed (-12 characters; 62 total characters):
// const isLE=new Uint8Array(new Uint16Array([256]).buffer)[0]<1;