refactor: extract trailing-slash helpers into url utils

src/utils/ajax.js

@@ -3,6 +3,7 @@
  */
 import { getGBKit } from './bridge';
 import { warn, debug } from './logger';
+import { stripTrailingSlash } from './url';
 
 /**
  * Configure AJAX for use without authentication cookies.
@@ -21,7 +22,7 @@ export function configureAjax() {
 	window.wp.ajax.settings = window.wp.ajax.settings || {};
 
 	const { siteURL: rawSiteURL, authHeader } = getGBKit();
-	const siteURL = rawSiteURL?.replace( /\/+$/, '' );
+	const siteURL = stripTrailingSlash( rawSiteURL );
 	configureAjaxUrl( siteURL );
 	configureAjaxAuth( siteURL, authHeader );
 	configureMediaAjax();

src/utils/api-fetch.js

@@ -10,6 +10,7 @@ import { __ } from '@wordpress/i18n';
  */
 import { getGBKit, POST_FALLBACKS } from './bridge';
 import { info, error as logError } from './logger';
+import { ensureTrailingSlash, stripTrailingSlash } from './url';
 
 /**
  * @typedef {import('@wordpress/api-fetch').APIFetchMiddleware} APIFetchMiddleware
@@ -85,7 +86,7 @@ function apiPathModifierMiddleware( options, next ) {
 		// single trailing slash.
 		options.path = options.path.replace(
 			/^(?<apiPath>\/?(?:[\w.-]+\/){2})/,
-			`$<apiPath>${ siteApiNamespace[ 0 ].replace( /\/+$/, '' ) }/`
+			`$<apiPath>${ ensureTrailingSlash( siteApiNamespace[ 0 ] ) }`
 		);
 	}
 
@@ -528,7 +529,7 @@ function siteIndexMiddleware( options, next ) {
 		return next( options );
 	}
 
-	const home = siteURL?.replace( /\/+$/, '' );
+	const home = stripTrailingSlash( siteURL );
 	return Promise.resolve( home ? { home } : {} );
 }
 

src/utils/url.js

@@ -0,0 +1,31 @@
+/**
+ * Removes any trailing slashes from a URL or path.
+ *
+ * An absent value normalizes to an empty string, so an unset config field can
+ * be handed over directly. Any other non-string still throws, so a wrong type
+ * is not quietly passed through.
+ *
+ * @param {string} [value] The URL or path to normalize.
+ * @return {string} The value without a trailing slash.
+ */
+export function stripTrailingSlash( value ) {
+	return ( value ?? '' ).replace( /\/+$/, '' );
+}
+
+/**
+ * Normalizes a URL or path to exactly one trailing slash.
+ *
+ * An absent value stays absent rather than becoming a lone slash, which would
+ * resolve requests against the page root. A value that is already just `/` is
+ * kept, since that is a meaningful root.
+ *
+ * @param {string} [value] The URL or path to normalize.
+ * @return {string} The value with a single trailing slash, or an empty string.
+ */
+export function ensureTrailingSlash( value ) {
+	if ( value === undefined || value === null || value === '' ) {
+		return '';
+	}
+
+	return `${ stripTrailingSlash( value ) }/`;
+}

src/utils/url.test.js

@@ -0,0 +1,49 @@
+/**
+ * External dependencies
+ */
+import { describe, it, expect } from 'vitest';
+
+/**
+ * Internal dependencies
+ */
+import { ensureTrailingSlash, stripTrailingSlash } from './url';
+
+describe( 'stripTrailingSlash', () => {
+	it.each( [
+		[ 'https://example.com/wp-json/', 'https://example.com/wp-json' ],
+		[ 'https://example.com/wp-json//', 'https://example.com/wp-json' ],
+		[ 'https://example.com/wp-json', 'https://example.com/wp-json' ],
+		[ 'sites/123/', 'sites/123' ],
+		[ '/', '' ],
+		[ '', '' ],
+		[ undefined, '' ],
+		[ null, '' ],
+	] )( 'normalizes %s to %s', ( value, expected ) => {
+		expect( stripTrailingSlash( value ) ).toBe( expected );
+	} );
+
+	// An absent value is expected; a wrong type is a caller bug worth surfacing.
+	it( 'throws on a non-string', () => {
+		expect( () => stripTrailingSlash( 42 ) ).toThrow( TypeError );
+	} );
+} );
+
+describe( 'ensureTrailingSlash', () => {
+	it.each( [
+		[ 'https://example.com/wp-json', 'https://example.com/wp-json/' ],
+		[ 'https://example.com/wp-json/', 'https://example.com/wp-json/' ],
+		[ 'https://example.com/wp-json//', 'https://example.com/wp-json/' ],
+		[ 'sites/123', 'sites/123/' ],
+		// A lone slash is a meaningful root, unlike an absent value.
+		[ '/', '/' ],
+		[ '', '' ],
+		[ undefined, '' ],
+		[ null, '' ],
+	] )( 'normalizes %s to %s', ( value, expected ) => {
+		expect( ensureTrailingSlash( value ) ).toBe( expected );
+	} );
+
+	it( 'throws on a non-string', () => {
+		expect( () => ensureTrailingSlash( 42 ) ).toThrow( TypeError );
+	} );
+} );