Wordpress - Does something like is_rest() exist

It's a good point by @Milo, the REST_REQUEST constant is defined as true, within rest_api_loaded() if $GLOBALS['wp']->query_vars['rest_route'] is non-empty.

It's hooked into parse_request via:

add_action( 'parse_request', 'rest_api_loaded' );

but parse_request fires later than init - See for example the Codex here.

There was a suggestion (by Daniel Bachhuber) in ticket #34373 regarding WP_Query::is_rest(), but it was postponed/cancelled.

Just stumbled over the same problem and wrote a simple function is_rest that allows you to check if the current request is a WP REST API request.

<?php
if ( !function_exists( 'is_rest' ) ) {
    /**
     * Checks if the current request is a WP REST API request.
     *
     * Case #1: After WP_REST_Request initialisation
     * Case #2: Support "plain" permalink settings
     * Case #3: It can happen that WP_Rewrite is not yet initialized,
     *          so do this (wp-settings.php)
     * Case #4: URL Path begins with wp-json/ (your REST prefix)
     *          Also supports WP installations in subfolders
     *
     * @returns boolean
     * @author matzeeable
     */
    function is_rest() {
        $prefix = rest_get_url_prefix( );
        if (defined('REST_REQUEST') && REST_REQUEST // (#1)
                || isset($_GET['rest_route']) // (#2)
                        && strpos( trim( $_GET['rest_route'], '\\/' ), $prefix , 0 ) === 0)
                return true;
        // (#3)
        global $wp_rewrite;
        if ($wp_rewrite === null) $wp_rewrite = new WP_Rewrite();

        // (#4)
        $rest_url = wp_parse_url( trailingslashit( rest_url( ) ) );
        $current_url = wp_parse_url( add_query_arg( array( ) ) );
        return strpos( $current_url['path'], $rest_url['path'], 0 ) === 0;
    }
}

References:

• https://gist.github.com/matzeeable/dfd82239f48c2fedef25141e48c8dc30
• https://twitter.com/matzeeable/status/1052967482737258496

Two options here really,

1. Check if REST_REQUEST is defined.
2. Hook into rest_api_init where you wanted to hook into init.