[javascript] Window.open and pass parameters by post method

I wanted to do this in React using plain Js and the fetch polyfill. OP didn't say he specifically wanted to create a form and invoke the submit method on it, so I have done it by posting the form values as json:

examplePostData = {
    method: 'POST',
    headers: {
       'Content-type' : 'application/json',
       'Accept' : 'text/html'
    },
    body: JSON.stringify({
        someList: [1,2,3,4],
        someProperty: 'something',
        someObject: {some: 'object'}
    })
}

asyncPostPopup = () => {

    //open a new window and set some text until the fetch completes
    let win=window.open('about:blank')
    writeToWindow(win,'Loading...')

    //async load the data into the window
    fetch('../postUrl', this.examplePostData)
    .then((response) => response.text())
    .then((text) => writeToWindow(win,text))
    .catch((error) => console.log(error))
}

writeToWindow = (win,text) => {
    win.document.open()
    win.document.write(text)
    win.document.close()
}