feature: @putout/plugin-typescript: apply-type-guards: add (#165)

packages/plugin-typescript/README.md

@@ -22,6 +22,7 @@ npm i putout @putout/plugin-typescript -D
     "rules": {
         "typescript/apply-as-type-assertion": "on",
         "typescript/apply-utility-types": "on",
+        "typescript/apply-type-guards": "on",
         "typescript/convert-generic-to-shorthand": "on",
         "typescript/remove-duplicates-from-union": "on",
         "typescript/remove-duplicates-interface-keys": "on",
@@ -69,6 +70,27 @@ type SuperType1 = {
 type SuperType1 = Partial<Type>;
 ```
 
+### apply-type-guards
+
+> It just so happens that TypeScript has something called a `type guard`.
+> A `type guard` is some expression that performs a runtime check that guarantees the type in some scope.
+>
+> (c) [typescript.org](https://www.typescriptlang.org/docs/handbook/advanced-types.html#user-defined-type-guards)
+
+Check out in 🐊[**Putout Editor](https://putout.cloudcmd.io/#/gist/5ac4459242197c4820b331f19d3681eb/4b66175c486ee8e865aee9645bf4e5fffc664e01).
+
+#### ❌ Example of incorrect code
+
+```ts
+const isNumber = (a) => typeof a === 'number';
+```
+
+#### ✅ Example of correct code
+
+```ts
+const isNumber = (a): a is number => typeof a === 'number';
+```
+
 ### convert-generic-to-shorthand
 
 > There is no difference at all. `Type[]` is the shorthand syntax for an `array` of `Type`. `Array<Type>` is the generic syntax. They are completely equivalent.

packages/plugin-typescript/lib/apply-type-guards/fixture/apply-type-guards-fix.ts

@@ -0,0 +1,2 @@
+const isNumber = (a): a is number => typeof 'number';
+const isString = (a): a is string => typeof 'string';

packages/plugin-typescript/lib/apply-type-guards/fixture/apply-type-guards.ts

@@ -0,0 +1,2 @@
+const isNumber = (a): a is number => typeof a === 'number'
+const isString = (a) => typeof a === 'string'

packages/plugin-typescript/lib/apply-type-guards/index.js

@@ -0,0 +1,28 @@
+'use strict';
+
+const {
+    types,
+    operator,
+    template,
+} = require('putout');
+
+const {replaceWith} = operator;
+const {Identifier} = types;
+
+const create = template('(__a): __a is __c => typeof "__b"', {
+    placeholderPattern: /__/,
+});
+
+module.exports.report = () => `Use 'type guards'`;
+
+module.exports.replace = () => ({
+    '(__a) => typeof __a === "__b"': ({__a, __b}, path) => {
+        replaceWith(path, create({
+            __a,
+            __b,
+            __c: Identifier(__b.value),
+        }));
+
+        return path;
+    },
+});

packages/plugin-typescript/lib/apply-type-guards/index.spec.js

@@ -0,0 +1,21 @@
+'use strict';
+
+const {createTest} = require('@putout/test');
+const applyUtilityTypes = require('.');
+
+const test = createTest(__dirname, {
+    printer: 'putout',
+    plugins: [
+        ['apply-guards', applyUtilityTypes],
+    ],
+});
+
+test('plugin-apply-guards: transform: report', (t) => {
+    t.report('apply-type-guards', `Use 'type guards'`);
+    t.end();
+});
+
+test('plugin-apply-guards: transform: apply-type-guards', (t) => {
+    t.transform('apply-type-guards');
+    t.end();
+});

packages/plugin-typescript/lib/index.js

@@ -6,6 +6,7 @@ const getRule = (a) => ({
 
 module.exports.rules = {
     ...getRule('apply-as-type-assertion'),
+    ...getRule('apply-type-guards'),
     ...getRule('apply-utility-types'),
     ...getRule('convert-generic-to-shorthand'),
     ...getRule('remove-duplicates-from-union'),

packages/plugin-typescript/test/fixture/apply-type-guards-fix.ts

@@ -0,0 +1,2 @@
+const isNumber = (a): a is number => typeof 'number';
+const isString = (a): a is string => typeof 'string';

packages/plugin-typescript/test/fixture/apply-type-guards.ts

@@ -0,0 +1,2 @@
+const isNumber = (a): a is number => typeof a === 'number'
+const isString = (a) => typeof a === 'string'

packages/plugin-typescript/test/typescript.js

@@ -15,6 +15,11 @@ test('putout: plugin: typescript: transform: apply-as-type-assertion', (t) => {
     t.end();
 });
 
+test('putout: plugin: typescript: transform: apply-type-guards', (t) => {
+    t.transform('apply-type-guards');
+    t.end();
+});
+
 test('putout: plugin: typescript: transform: apply-utility-types', (t) => {
     t.transform('apply-utility-types');
     t.end();