Headers: has() method

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since March 2017.

Note: This feature is available in Web Workers.

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.

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:

const 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 # ref-for-dom-headers-has①

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