WordPress at a glance
function is not described

wp_is_json_request() WP 5.0.0

Checks whether current request is a JSON request, or is expecting a JSON response.

No Hooks.

Return

true/false. True if Accepts or Content-Type headers contain application/json, false otherwise.

Usage

wp_is_json_request();

Code of wp is json request: wp-includes/load.php VER 5.1.1

<?php
function wp_is_json_request() {

	if ( isset( $_SERVER['HTTP_ACCEPT'] ) && false !== strpos( $_SERVER['HTTP_ACCEPT'], 'application/json' ) ) {
		return true;
	}

	if ( isset( $_SERVER['CONTENT_TYPE'] ) && 'application/json' === $_SERVER['CONTENT_TYPE'] ) {
		return true;
	}

	return false;

}