JavaScript fetch APIを書き換えて通信内容を変更する

fetch.js

(() => {
	const isTarget = response => response.url.startsWith("http://example.com/path");
	const changeJson = json => {
		// jsonを書き変える
		return json;
	};

	const originalFetch = window.fetch;

	window.fetch = (...args) => {
		const promise = originalFetch(...args);
		const originalThen = promise.then;
		promise.then = (onFulfilled) => {
			return originalThen.call(promise, (response) => {
				const originalJson = response.json;
				response.json = () => {
					return originalJson.call(response).then(json => {
						if (isTarget(response)) {
							json = changeJson(json);
						}
						return json;
					});
				};
				return onFulfilled(response);
			});
		};
		return promise;
	};
})();

TODO: 第2引数 onRejected 対応

Promise.prototype.then() - JavaScript | MDN