feat: add prefer-get-or-insert

README.md

@@ -149,6 +149,7 @@ Read more at the
 | [prefer-object-has-own](./src/rules/prefer-object-has-own.ts) | Prefer `Object.hasOwn()` over `Object.prototype.hasOwnProperty.call()` and `obj.hasOwnProperty()` | ✅ | ✅ | ✖️ |
 | [prefer-spread-syntax](./src/rules/prefer-spread-syntax.ts) | Prefer spread syntax over `Array.concat()`, `Array.from()`, `Object.assign({}, ...)`, and `Function.apply()` | ✅ | ✅ | 🔶 |
 | [prefer-url-canparse](./src/rules/prefer-url-canparse.ts) | Prefer `URL.canParse()` over try-catch blocks for URL validation | ✅ | 💡 | ✖️ |
+| [prefer-get-or-insert](./src/rules/prefer-get-or-insert.ts) | Prefer `Map.prototype.getOrInsert()` over reading a map entry with a default and writing it back | ✖️ | ✅ | ✖️ |
 
 ### Module replacements
 

src/main.ts

@@ -29,6 +29,7 @@ import {preferStringFromCharCode} from './rules/prefer-string-fromcharcode.js';
 import {noSpreadInReduce} from './rules/no-spread-in-reduce.js';
 import {preferFlatMapOverMapFlat} from './rules/prefer-flatmap-over-map-flat.js';
 import {preferStaticCollator} from './rules/prefer-static-collator.js';
+import {preferGetOrInsert} from './rules/prefer-get-or-insert.js';
 
 const plugin: ESLint.Plugin = {
   meta: {
@@ -62,6 +63,7 @@ const plugin: ESLint.Plugin = {
     'no-spread-in-reduce': noSpreadInReduce,
     'prefer-flatmap-over-map-flat': preferFlatMapOverMapFlat,
     'prefer-static-collator': preferStaticCollator,
+    'prefer-get-or-insert': preferGetOrInsert,
     'ban-dependencies': banDependencies
   }
 };

src/rules/prefer-get-or-insert.test.ts

@@ -0,0 +1,130 @@
+import {RuleTester} from 'eslint';
+import {preferGetOrInsert} from './prefer-get-or-insert.js';
+
+const ruleTester = new RuleTester({
+  languageOptions: {
+    ecmaVersion: 2022,
+    sourceType: 'module'
+  }
+});
+
+ruleTester.run('prefer-get-or-insert', preferGetOrInsert, {
+  valid: [
+    // plain get/set, no default
+    'map.set(k, map.get(k));',
+
+    // setting an unrelated value
+    'map.set(k, other.get(k) ?? def);',
+    'map.set(k, map.get(other) ?? def);',
+
+    // different map between set and get
+    'map.set(k, other.has(k) ? other.get(k) : def);',
+
+    // ternary uses a different key for has vs get
+    'map.set(k, map.has(k) ? map.get(other) : def);',
+
+    // read-with-default that is never written back
+    'const v = map.get(k) ?? def; use(v);',
+    'const v = map.has(k) ? map.get(k) : def; use(v);',
+
+    // written back to a different map / key
+    'const v = map.get(k) ?? def; other.set(k, v);',
+    'const v = map.get(k) ?? def; map.set(other, v);',
+
+    // written back, but a different variable
+    'const v = map.get(k) ?? def; map.set(k, somethingElse);',
+
+    // set in a different block than the declaration
+    'const v = map.get(k) ?? def; if (x) { map.set(k, v); }',
+
+    // computed / optional access is out of scope
+    'map["set"](k, map.get(k) ?? def);',
+    'map?.set(k, map.get(k) ?? def);',
+
+    // logical operators other than ??
+    'map.set(k, map.get(k) || def);',
+
+    // guarded lazy-init that never writes the default back
+    'let v = map.get(k); if (!v) { v = def; }',
+
+    // guarded lazy-init writing back to a different map / key
+    'let v = map.get(k); if (!v) { v = def; other.set(k, v); }',
+    'let v = map.get(k); if (!v) { v = def; map.set(other, v); }'
+  ],
+
+  invalid: [
+    // map.set(k, map.get(k) ?? default)
+    {
+      code: 'map.set(k, map.get(k) ?? def);',
+      output: 'map.getOrInsert(k, def);',
+      errors: [{messageId: 'preferGetOrInsert'}]
+    },
+    // member-expression key
+    {
+      code: 'cache.set(obj.id, cache.get(obj.id) ?? createDefault());',
+      output: 'cache.getOrInsert(obj.id, createDefault());',
+      errors: [{messageId: 'preferGetOrInsert'}]
+    },
+    // map.set(k, map.has(k) ? map.get(k) : default)
+    {
+      code: 'map.set(k, map.has(k) ? map.get(k) : []);',
+      output: 'map.getOrInsert(k, []);',
+      errors: [{messageId: 'preferGetOrInsert'}]
+    },
+    // member-expression receiver
+    {
+      code: 'this.cache.set(k, this.cache.get(k) ?? def);',
+      output: 'this.cache.getOrInsert(k, def);',
+      errors: [{messageId: 'preferGetOrInsert'}]
+    },
+    // const v = map.get(k) ?? default; ...; map.set(k, v)
+    {
+      code: `
+        const v = map.get(k) ?? def;
+        v.push(1);
+        map.set(k, v);
+      `,
+      output: `
+        const v = map.getOrInsert(k, def);
+        v.push(1);
+      `,
+      errors: [{messageId: 'preferGetOrInsert'}]
+    },
+    // const v = map.has(k) ? map.get(k) : default; ...; map.set(k, v)
+    {
+      code: `
+        let v = map.has(k) ? map.get(k) : [];
+        map.set(k, v);
+      `,
+      output: `
+        let v = map.getOrInsert(k, []);
+      `,
+      errors: [{messageId: 'preferGetOrInsert'}]
+    },
+    // reassignment instead of declaration
+    {
+      code: `
+        v = map.get(k) ?? def;
+        map.set(k, v);
+      `,
+      output: `
+        v = map.getOrInsert(k, def);
+      `,
+      errors: [{messageId: 'preferGetOrInsert'}]
+    },
+    // let v = map.get(k); if (!v) { v = default; map.set(k, v); }
+    {
+      code: `
+        let v = map.get(k);
+        if (!v) {
+          v = [];
+          map.set(k, v);
+        }
+      `,
+      output: `
+        let v = map.getOrInsert(k, []);
+      `,
+      errors: [{messageId: 'preferGetOrInsert'}]
+    }
+  ]
+});