From 1018c2029eea1f5b75b5120265996f1c0b3c12ae Mon Sep 17 00:00:00 2001
From: Szymon Marczak <36894700+szmarczak@users.noreply.github.com>
Date: Fri, 30 Jul 2021 14:09:21 +0200
Subject: [PATCH] Fix merging `searchParams`

Fixes #1814
---
 documentation/2-options.md  |  2 +-
 source/core/options.ts      | 32 +++++++++++++++++++++-----------
 test/normalize-arguments.ts | 17 +++++++++++++++++
 3 files changed, 39 insertions(+), 12 deletions(-)

diff --git a/documentation/2-options.md b/documentation/2-options.md
index 989072daa..0ac3faacb 100644
--- a/documentation/2-options.md
+++ b/documentation/2-options.md
@@ -177,7 +177,7 @@ console.log(searchParams.toString());
 
 #### **Note:**
 > - `null` values are not stringified, an empty string is used instead.
-> - `undefined` values are skipped.
+> - `undefined` values will clear the original keys.
 
 #### **Merge behavior:**
 > - Overrides existing properties.
diff --git a/source/core/options.ts b/source/core/options.ts
index a013d59d4..556744dc5 100644
--- a/source/core/options.ts
+++ b/source/core/options.ts
@@ -1325,6 +1325,10 @@ export default class Options {
 			return (this._internals.url as URL).searchParams;
 		}
 
+		if (this._internals.searchParams === undefined) {
+			this._internals.searchParams = new URLSearchParams();
+		}
+
 		return this._internals.searchParams;
 	}
 
@@ -1343,11 +1347,13 @@ export default class Options {
 			return;
 		}
 
-		let searchParameters = (this.searchParams ?? new URLSearchParams()) as URLSearchParams;
+		const searchParameters = this.searchParams as URLSearchParams;
 		let updated;
 
-		if (is.string(value) || (value instanceof URLSearchParams)) {
+		if (is.string(value)) {
 			updated = new URLSearchParams(value);
+		} else if (value instanceof URLSearchParams) {
+			updated = value;
 		} else {
 			validateSearchParameters(value);
 
@@ -1359,22 +1365,26 @@ export default class Options {
 
 				if (entry === null) {
 					updated.append(key, '');
-				} else if (entry !== undefined) {
+				} else if (entry === undefined) {
+					searchParameters.delete(key);
+				} else {
 					updated.append(key, entry as string);
 				}
 			}
 		}
 
 		if (this._merging) {
-			// eslint-disable-next-line unicorn/no-array-for-each
-			updated.forEach((value, key) => {
-				searchParameters.set(key, value);
-			});
-		} else {
-			searchParameters = updated;
-		}
+			// These keys will be replaced
+			for (const key of updated.keys()) {
+				searchParameters.delete(key);
+			}
 
-		if (!url) {
+			for (const [key, value] of updated) {
+				searchParameters.append(key, value);
+			}
+		} else if (url) {
+			url.search = searchParameters.toString();
+		} else {
 			this._internals.searchParams = searchParameters;
 		}
 	}
diff --git a/test/normalize-arguments.ts b/test/normalize-arguments.ts
index 096d141b8..4930a0ec7 100644
--- a/test/normalize-arguments.ts
+++ b/test/normalize-arguments.ts
@@ -150,3 +150,20 @@ test('extending responseType', t => {
 
 	t.is(merged.defaults.options.responseType, 'json');
 });
+
+test('searchParams - multiple values for one key', t => {
+	const searchParams = new URLSearchParams();
+
+	searchParams.append('a', '100');
+	searchParams.append('a', '200');
+	searchParams.append('a', '300');
+
+	const options = new Options({
+		searchParams
+	});
+
+	t.deepEqual(
+		(options.searchParams as URLSearchParams).getAll('a'),
+		['100', '200', '300']
+	);
+});