Example manual implementation of fetch().then(response => response.arrayBuffer())

fetch-response-arraybuffer-implementation.js

// Body mixin arrayBuffer helper.
return response.arrayBuffer();

// Example implementation of arrayBuffer().
const reader = response.body.getReader();
return new Promise((resolve, reject) => {
    const chunks = [];
    const step = function () {
        reader.read().then(({done, value}) => {
            if (done) {
                const uint8 = new Uint8Array(chunks.reduce((carry, chunk) => carry + chunk.length, 0));
                let position = 0;
                for (let i = 0; i < chunks.length; i++) {
                    uint8.set(chunks[i], position);
                    position += chunks[i].length;
                }
                resolve(uint8.buffer);
            } else {
                chunks.push(value);
                setTimeout(step);
            }
        }, reject);
    };
    setTimeout(step);
});