Headers.has()

The has() method of the Headers interface returns a boolean stating whether a Headers object contains a certain header.

For security reasons, some headers can only be controlled by the user agent. These headers include the forbidden header names and forbidden response header names.

Syntax

myHeaders.has(name);

name: The name of the HTTP header you want to test for. If the given name is not a valid HTTP header name, this method throws a TypeError.

A boolean value.

Creating an empty Headers object is simple:

var myHeaders = new Headers(); // Currently empty

You could add a header to this using Headers.append, then test for the existence of it using has():

myHeaders.append('Content-Type', 'image/jpeg');
myHeaders.has('Content-Type'); // Returns true
myHeaders.has('Accept-Encoding'); // Returns false

Specification
Fetch Standard (Fetch) # ref-for-dom-headers-has①

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	WebView Android	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet
`has`	42	14	39	No	29	10.1	42	42	No	29	10.3	4.0