XMLHttpRequest: open() method

Note: This feature is available in Web Workers, except for Service Workers.

The XMLHttpRequest method open() initializes a newly-created request, or re-initializes an existing one.

Note: Calling this method for an already active request (one for which open() has already been called) is the equivalent of calling abort().

Syntax

open(method, url)
open(method, url, async)
open(method, url, async, user)
open(method, url, async, user, password)

method: The HTTP request method to use, such as "GET", "POST", "PUT", "DELETE", etc. Ignored for non-HTTP(S) URLs.
url: A string or any other object with a stringifier — including a URL object — that provides the URL of the resource to send the request to.
async Optional: An optional Boolean parameter, defaulting to true, indicating whether or not to perform the operation asynchronously. If this value is false, the send() method does not return until the response is received. If true, notification of a completed transaction is provided using event listeners. This must be true if the multipart attribute is true, or an exception will be thrown.

Note: Synchronous requests on the main thread can be easily disruptive to the user experience and should be avoided; in fact, many browsers have deprecated synchronous XHR support on the main thread entirely. Synchronous requests are permitted in Workers.
user Optional: The optional user name to use for authentication purposes; by default, this is the null value.
password Optional: The optional password to use for authentication purposes; by default, this is the null value.

Specification
XMLHttpRequest Standard # the-open()-method

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