fix: uneval emits valid JS for repeated empty Map/Set (#168)

* fix: uneval emits valid JS for repeated empty Map/Set

A Map or Set referenced more than once is bound to a variable and populated via
a chained statement (e.g. a.set(...).set(...)). When the collection was empty,
this produced a dangling member expression 'a.' \u2014 a syntax error. Emit no
chained statement for empty collections; 'new Map'/'new Set' already builds them.

* refactor: address review feedback on Map/Set chain building

Author: elliott-with-the-longest-name-on-github

.changeset/fix-empty-map-set-uneval.md

@@ -0,0 +1,5 @@
+---
+"devalue": patch
+---
+
+fix: `uneval` now produces valid output for a repeated empty `Map` or `Set`

src/uneval.js

@@ -413,23 +413,23 @@ export function uneval(value, replacer) {
 					});
 					break;
 
-				case 'Set':
+				case 'Set': {
 					values.push(`new Set`);
-					statements.push(
-						`${name}.${Array.from(thing)
-							.map((v) => `add(${stringify(v)})`)
-							.join('.')}`
-					);
+					const adds = Array.from(thing).map((v) => `.add(${stringify(v)})`);
+					// An empty Set is fully built by `new Set`; a chained statement would
+					// otherwise be a dangling `name.`.
+					if (adds.length > 0) statements.push(name + adds.join(''));
 					break;
+				}
 
-				case 'Map':
+				case 'Map': {
 					values.push(`new Map`);
-					statements.push(
-						`${name}.${Array.from(thing)
-							.map(([k, v]) => `set(${stringify(k)}, ${stringify(v)})`)
-							.join('.')}`
+					const sets = Array.from(thing).map(
+						([k, v]) => `.set(${stringify(k)}, ${stringify(v)})`
 					);
+					if (sets.length > 0) statements.push(name + sets.join(''));
 					break;
+				}
 
 				case 'Int8Array':
 				case 'Uint8Array':

test/index.test.js

@@ -630,6 +630,28 @@ const fixtures = {
 			validate: ([a, b]) => assert.is(a, b)
 		},
 
+		{
+			name: 'empty Map (repetition)',
+			value: ((map) => [map, map])(new Map()),
+			js: '(function(a){return [a,a]}(new Map))',
+			json: '[[1,1],["Map"]]',
+			validate: ([a, b]) => {
+				assert.is(a, b);
+				assert.is(a.size, 0);
+			}
+		},
+
+		{
+			name: 'empty Set (repetition)',
+			value: ((set) => [set, set])(new Set()),
+			js: '(function(a){return [a,a]}(new Set))',
+			json: '[[1,1],["Set"]]',
+			validate: ([a, b]) => {
+				assert.is(a, b);
+				assert.is(a.size, 0);
+			}
+		},
+
 		{
 			name: 'RegExp (repetition)',
 			value: ((regexp) => [regexp, regexp])(/regexp/),