From ee368549cdf193debbc81c93677be3583dc6e3c9 Mon Sep 17 00:00:00 2001
From: Jarda Snajdr <jsnajdr@gmail.com>
Date: Wed, 2 Sep 2020 10:50:43 +0200
Subject: [PATCH] api-fetch: simplify the code that executes the handlers
 (#24999)

* apiFetch: simplify the promise-chaining code

* Use reduce(Right) to compose middlewares
---
 packages/api-fetch/src/index.js | 48 ++++++++++++++-------------------
 1 file changed, 20 insertions(+), 28 deletions(-)

diff --git a/packages/api-fetch/src/index.js b/packages/api-fetch/src/index.js
index b9fe1a6d0875aa..a77a6295637a96 100644
--- a/packages/api-fetch/src/index.js
+++ b/packages/api-fetch/src/index.js
@@ -120,36 +120,28 @@ function setFetchHandler( newFetchHandler ) {
 }
 
 function apiFetch( options ) {
-	const steps = [ ...middlewares, fetchHandler ];
-
-	const createRunStep = ( index ) => ( workingOptions ) => {
-		const step = steps[ index ];
-		if ( index === steps.length - 1 ) {
-			return step( workingOptions );
+	// creates a nested function chain that calls all middlewares and finally the `fetchHandler`,
+	// converting `middlewares = [ m1, m2, m3 ]` into:
+	// ```
+	// opts1 => m1( opts1, opts2 => m2( opts2, opts3 => m3( opts3, fetchHandler ) ) );
+	// ```
+	const enhancedHandler = middlewares.reduceRight( ( next, middleware ) => {
+		return ( workingOptions ) => middleware( workingOptions, next );
+	}, fetchHandler );
+
+	return enhancedHandler( options ).catch( ( error ) => {
+		if ( error.code !== 'rest_cookie_invalid_nonce' ) {
+			return Promise.reject( error );
 		}
 
-		const next = createRunStep( index + 1 );
-		return step( workingOptions, next );
-	};
-
-	return new Promise( ( resolve, reject ) => {
-		createRunStep( 0 )( options )
-			.then( resolve )
-			.catch( ( error ) => {
-				if ( error.code !== 'rest_cookie_invalid_nonce' ) {
-					return reject( error );
-				}
-
-				// If the nonce is invalid, refresh it and try again.
-				window
-					.fetch( apiFetch.nonceEndpoint )
-					.then( checkStatus )
-					.then( ( data ) => data.text() )
-					.then( ( text ) => {
-						apiFetch.nonceMiddleware.nonce = text;
-						apiFetch( options ).then( resolve ).catch( reject );
-					} )
-					.catch( reject );
+		// If the nonce is invalid, refresh it and try again.
+		return window
+			.fetch( apiFetch.nonceEndpoint )
+			.then( checkStatus )
+			.then( ( data ) => data.text() )
+			.then( ( text ) => {
+				apiFetch.nonceMiddleware.nonce = text;
+				return apiFetch( options );
 			} );
 	} );
 }