mock fetch calls, overriding the response (e.g. Server down during development)

mockFetch.js

// e.g. mockFetch({'avengers/me': {'name': "iron man", 'rank': 2}})

function mockFetch(urlResponseMap) {
    
    window.fetchOriginal = window.fetch
    window.fetch = (...params) => {
    const lookFor = Object.keys(urlResponseMap);

    const url = typeof params[0] == 'object' ? params[0].url : params[0];

    lookFor.forEach(str => {
        const returnObj = urlResponseMap[str];

        if (url.includes(str)) {
          console.log("returning mock response for ", str)
          return new Promise(() => (
            {
                ...returnObj,
                toString: () => (returnObj),
                json: () => (returnObj)

            }
        ))
        }
        else {
          const arg = typeof params == 'object' ? Object.assign({}, params) : params;
          window.fetch = window.fetchOriginal;
          window.fetchOriginal.apply(window, arg)
        }
    })
      
    }
}