typescript-eslint · JamesHenry · Jul 24, 2019 · Jun 1, 2019 · Jun 1, 2019 · Jun 1, 2019
diff --git a/packages/eslint-plugin/README.md b/packages/eslint-plugin/README.md
@@ -181,6 +181,7 @@ Then you should add `airbnb` (or `airbnb-base`) to your `extends` section of `.e
 | [`@typescript-eslint/strict-boolean-expressions`](./docs/rules/strict-boolean-expressions.md)             | Restricts the types allowed in boolean expressions                                                                                                  |                    |          | :thought_balloon: |
 | [`@typescript-eslint/triple-slash-reference`](./docs/rules/triple-slash-reference.md)                     | Sets preference level for triple slash directives versus ES6-style import declarations                                                              |                    |          |                   |
 | [`@typescript-eslint/type-annotation-spacing`](./docs/rules/type-annotation-spacing.md)                   | Require consistent spacing around type annotations                                                                                                  | :heavy_check_mark: | :wrench: |                   |
+| [`@typescript-eslint/typedef`](./docs/rules/typedef.md)                                                   | Requires type annotations to exist                                                                                                                  |                    |          |                   |
 | [`@typescript-eslint/unbound-method`](./docs/rules/unbound-method.md)                                     | Enforces unbound methods are called with their expected scope                                                                                       |                    |          | :thought_balloon: |
 | [`@typescript-eslint/unified-signatures`](./docs/rules/unified-signatures.md)                             | Warns for any two overloads that could be unified into one by using a union or an optional/rest parameter                                           |                    |          |                   |
 

diff --git a/packages/eslint-plugin/ROADMAP.md b/packages/eslint-plugin/ROADMAP.md
@@ -32,8 +32,8 @@
 | [`only-arrow-functions`]          | 🔌  | [`prefer-arrow/prefer-arrow-functions`]              |
 | [`prefer-for-of`]                 | ✅  | [`@typescript-eslint/prefer-for-of`]                 |
 | [`promise-function-async`]        | ✅  | [`@typescript-eslint/promise-function-async`]        |
-| [`typedef`]                       | 🛑  | N/A                                                  |
 | [`typedef-whitespace`]            | ✅  | [`@typescript-eslint/type-annotation-spacing`]       |
+| [`typedef`]                       | ✅  | [`@typescript-eslint/typedef`]                       |
 | [`unified-signatures`]            | ✅  | [`@typescript-eslint/unified-signatures`]            |
 
 <sup>[1]</sup> The ESLint rule only supports exact string matching, rather than regular expressions<br>
@@ -592,6 +592,7 @@ Relevant plugins: [`chai-expect-keywords`](https://github.com/gavinaiken/eslint-
 [`@typescript-eslint/no-unnecessary-type-assertion`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/no-unnecessary-type-assertion.md
 [`@typescript-eslint/no-var-requires`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/no-var-requires.md
 [`@typescript-eslint/type-annotation-spacing`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/type-annotation-spacing.md
+[`@typescript-eslint/typedef`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/typedef.md
 [`@typescript-eslint/unified-signatures`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/unified-signatures.md
 [`@typescript-eslint/no-misused-new`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/no-misused-new.md
 [`@typescript-eslint/no-object-literal-type-assertion`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/no-object-literal-type-assertion.md

diff --git a/packages/eslint-plugin/docs/rules/typedef.md b/packages/eslint-plugin/docs/rules/typedef.md
@@ -0,0 +1,264 @@
+# Require type annotations to exist (typedef)
+
+TypeScript cannot always infer types for all places in code.
+Some locations require type annotations for their types to be inferred.
+
+```ts
+class ContainsText {
+  // There must be a type annotation here to infer the type
+  delayedText: string;
+
+  // `typedef` requires a type annotation here to maintain consistency
+  immediateTextExplicit: string = 'text';
+
+  // This is still a string type because of its initial value
+  immediateTextImplicit = 'text';
+}
+```
+
+> Note: requiring type annotations unnecessarily can be cumbersome to maintain and generally reduces code readability.
+> TypeScript is often better at inferring types than easily written type annotations would allow.
+> Instead of enabling `typedef`, it is generally recommended to use the `--noImplicitAny` and/or `--strictPropertyInitialization` compiler options to enforce type annotations only when useful.
+
+## Rule Details
+
+This rule can enforce type annotations in locations regardless of whether they're required.
+This is typically used to maintain consistency for element types that sometimes require them.
+
+> To enforce type definitions existing on call signatures as per TSLint's `arrow-call-signature` and `call-signature` options, use `explicit-function-return-type`.
+
+## Options
+
+This rule has an object option that may receive any of the following as booleans:
+
+- `"arrayDestructuring"`
+- `"arrowParameter"`: `true` by default
+- `"memberVariableDeclaration"`: `true` by default
+- `"objectDestructuring"`
+- `"parameter"`: `true` by default
+- `"propertyDeclaration"`: `true` by default
+- `"variableDeclaration"`
+
+For example, with the following configuration:
+
+```json
+{
+  "rules": {
+    "typedef": [
+      "error",
+      {
+        "arrowParameter": false,
+        "variableDeclaration": true
+      }
+    ]
+  }
+}
+```
+
+- Type annotations on arrow function parameters are not required
+- Type annotations on variables are required
+- Options otherwise adhere to the defaults
+
+### arrayDestructuring
+
+Whether to enforce type annotations on variables declared using array destructuring.
+
+Examples of **incorrect** code with `{ "arrayDestructuring": true }`:
+
+```ts
+const [a] = [1];
+const [b, c] = [1, 2];
+```
+
+Examples of **correct** code with `{ "arrayDestructuring": true }`:
+
+```ts
+const [a]: number[] = [1];
+const [b]: [number] = [2];
+const [c, d]: [boolean, string] = [true, 'text'];
+```
+
+### arrowParameter
+
+Whether to enforce type annotations for parameters of arrow functions.
+
+Examples of **incorrect** code with `{ "arrowParameter": true }`:
+
+```ts
+const logsSize = size => console.log(size);
+
+['hello', 'world'].map(text => text.length);
+
+const mapper = {
+  map: text => text + '...',
+};
+```
+
+Examples of **correct** code with `{ "arrowParameter": true }`:
+
+```ts
+const logsSize = (size: number) => console.log(text);
+
+['hello', 'world'].map((text: string) => text.length);
+
+const mapper = {
+  map: (text: string) => text + '...',
+};
+```
+
+### memberVariableDeclaration
+
+Whether to enforce type annotations on member variables of classes.
+
+Examples of **incorrect** code with `{ "memberVariableDeclaration": true }`:
+
+```ts
+class ContainsText {
+  delayedText;
+  immediateTextImplicit = 'text';
+}
+```
+
+Examples of **correct** code with `{ "memberVariableDeclaration": true }`:
+
+```ts
+class ContainsText {
+  delayedText: string;
+  immediateTextImplicit: string = 'text';
+}
+```
+
+### objectDestructuring
+
+Whether to enforce type annotations on variables declared using object destructuring.
+
+Examples of **incorrect** code with `{ "objectDestructuring": true }`:
+
+```ts
+const { length } = 'text';
+const [b, c] = Math.random() ? [1, 2] : [3, 4];
+```
+
+Examples of **correct** code with `{ "objectDestructuring": true }`:
+
+```ts
+const { length }: { length: number } = 'text';
+const [b, c]: [number, number] = Math.random() ? [1, 2] : [3, 4];
+```
+
+### parameter
+
+Whether to enforce type annotations for parameters of functions and methods.
+
+Examples of **incorrect** code with `{ "parameter": true }`:
+
+```ts
+function logsSize(size): void {
+  console.log(size);
+}
+
+const doublesSize = function(size): numeber {
+  return size * 2;
+};
+
+const divider = {
+  curriesSize(size): number {
+    return size;
+  },
+  dividesSize: function(size): number {
+    return size / 2;
+  },
+};
+
+class Logger {
+  log(text): boolean {
+    console.log('>', text);
+    return true;
+  }
+}
+```
+
+Examples of **correct** code with `{ "parameter": true }`:
+
+```ts
+function logsSize(size: number): void {
+  console.log(size);
+}
+
+const doublesSize = function(size: number): numeber {
+  return size * 2;
+};
+
+const divider = {
+  curriesSize(size: number): number {
+    return size;
+  },
+  dividesSize: function(size: number): number {
+    return size / 2;
+  },
+};
+
+class Logger {
+  log(text: boolean): boolean {
+    console.log('>', text);
+    return true;
+  }
+}
+```
+
+### propertyDeclaration
+
+Whether to enforce type annotations for properties of interfaces and types.
+
+Examples of **incorrect** code with `{ "propertyDeclaration": true }`:
+
+```ts
+type Members = {
+  member;
+  otherMember;
+};
+```
+
+Examples of **correct** code with `{ "propertyDeclaration": true }`:
+
+```ts
+type Members = {
+  member: boolean;
+  otherMember: string;
+};
+```
+
+### variableDeclaration
+
+Whether to enforce type annotations for variable declarations, excluding array and object destructuring.
+
+Examples of **incorrect** code with `{ "variableDeclaration": true }`:
+
+```ts
+const text = 'text';
+let initialText = 'text';
+let delayedText;
+```
+
+Examples of **correct** code with `{ "variableDeclaration": true }`:
+
+```ts
+const text: string = 'text';
+let initialText: string = 'text';
+let delayedText: string;
+```
+
+## When Not To Use It
+
+If you are using stricter TypeScript compiler options, particularly `--noImplicitAny` and/or `--strictPropertyInitialization`, you likely don't need this rule.
+
+In general, if you do not consider the cost of writing unnecessary type annotations reasonable, then do not use this rule.
+
+## Further Reading
+
+- [TypeScript Type System](https://basarat.gitbooks.io/typescript/docs/types/type-system.html)
+- [Type Inference](https://www.typescriptlang.org/docs/handbook/type-inference.html)
+
+## Compatibility
+
+- TSLint: [typedef](https://palantir.github.io/tslint/rules/typedef)
diff --git a/packages/eslint-plugin/src/configs/all.json b/packages/eslint-plugin/src/configs/all.json
@@ -69,6 +69,7 @@
     "@typescript-eslint/strict-boolean-expressions": "error",
     "@typescript-eslint/triple-slash-reference": "error",
     "@typescript-eslint/type-annotation-spacing": "error",
+    "@typescript-eslint/typedef": "error",
     "@typescript-eslint/unbound-method": "error",
     "@typescript-eslint/unified-signatures": "error"
   }

diff --git a/packages/eslint-plugin/src/rules/index.ts b/packages/eslint-plugin/src/rules/index.ts
@@ -56,6 +56,7 @@ import semi from './semi';
 import strictBooleanExpressions from './strict-boolean-expressions';
 import tripleSlashReference from './triple-slash-reference';
 import typeAnnotationSpacing from './type-annotation-spacing';
+import typedef from './typedef';
 import unboundMethod from './unbound-method';
 import unifiedSignatures from './unified-signatures';
 
@@ -114,10 +115,11 @@ export default {
   'require-array-sort-compare': requireArraySortCompare,
   'require-await': requireAwait,
   'restrict-plus-operands': restrictPlusOperands,
-  semi: semi,
   'strict-boolean-expressions': strictBooleanExpressions,
   'triple-slash-reference': tripleSlashReference,
   'type-annotation-spacing': typeAnnotationSpacing,
   'unbound-method': unboundMethod,
   'unified-signatures': unifiedSignatures,
+  semi: semi,
+  typedef: typedef,
 };