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XMLHttpRequest: readyState property

The XMLHttpRequest.readyState property returns the state an XMLHttpRequest client is in. An XHR client exists in one of the following states:

Value	State	Description
`0`	`UNSENT`	Client has been created. `open()` not called yet.
`1`	`OPENED`	`open()` has been called.
`2`	`HEADERS_RECEIVED`	`send()` has been called, and headers and status are available.
`3`	`LOADING`	Downloading; `responseText` holds partial data.
`4`	`DONE`	The operation is complete.

UNSENT: The XMLHttpRequest client has been created, but the open() method hasn't been called yet.
OPENED: open() method has been invoked. During this state, the request headers can be set using the setRequestHeader() method and the send() method can be called which will initiate the fetch.
HEADERS_RECEIVED: send() has been called, all redirects (if any) have been followed and the response headers have been received.
LOADING: Response's body is being received. If responseType is "text" or empty string, responseText will have the partial text response as it loads.
DONE: The fetch operation is complete. This could mean that either the data transfer has been completed successfully or failed.

Example

const xhr = new XMLHttpRequest();
console.log("UNSENT", xhr.readyState); // readyState will be 0

xhr.open("GET", "/api", true);
console.log("OPENED", xhr.readyState); // readyState will be 1

xhr.onprogress = () => {
  console.log("LOADING", xhr.readyState); // readyState will be 3
};

xhr.onload = () => {
  console.log("DONE", xhr.readyState); // readyState will be 4
};

xhr.send(null);

Specifications

Specification
XMLHttpRequest Standard # states

Browser compatibility

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	WebView Android	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet
`readyState`	1	12	1	7	8	1.2	4.4	18	4	10.1	1	1.0

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https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/readyState