Add "no-null-undefined-union" rule.

src/configs/all.ts

@@ -126,6 +126,7 @@ export const rules = {
     // "no-invalid-this": Won't this be deprecated?
     "no-misused-new": true,
     "no-null-keyword": true,
+    "no-null-undefined-union": true,
     "no-object-literal-type-assertion": true,
     "no-return-await": true,
     "no-shadowed-variable": true,

src/configuration.ts

@@ -378,6 +378,7 @@ export function extendConfigurationFile(
  *
  * @deprecated use `path.resolve` instead
  */
+// tslint:disable-next-line no-null-undefined-union
 export function getRelativePath(directory?: string | null, relativeTo?: string) {
     if (directory != undefined) {
         const basePath = relativeTo !== undefined ? relativeTo : process.cwd();
@@ -428,6 +429,7 @@ export function getRulesDirectories(
  * @param ruleConfigValue The raw option setting of a rule
  */
 function parseRuleOptions(
+    // tslint:disable-next-line no-null-undefined-union
     ruleConfigValue: RawRuleConfig,
     rawDefaultRuleSeverity: string | undefined,
 ): Partial<IOptions> {
@@ -506,8 +508,11 @@ export interface RawConfigFile {
     jsRules?: RawRulesConfig | boolean;
 }
 export interface RawRulesConfig {
+    // tslint:disable-next-line no-null-undefined-union
     [key: string]: RawRuleConfig;
 }
+
+// tslint:disable-next-line no-null-undefined-union
 export type RawRuleConfig =
     | null
     | undefined

src/rules/noNullUndefinedUnionRule.ts

@@ -0,0 +1,105 @@
+/**
+ * @license
+ * Copyright 2019 Palantir Technologies, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import {
+    isParameterDeclaration,
+    isPropertyDeclaration,
+    isPropertySignature,
+    isSignatureDeclaration,
+    isTypeAliasDeclaration,
+    isTypeReference,
+    isUnionType,
+    isVariableDeclaration,
+} from "tsutils";
+import * as ts from "typescript";
+
+import * as Lint from "../index";
+
+export class Rule extends Lint.Rules.TypedRule {
+    /* tslint:disable:object-literal-sort-keys */
+    public static metadata: Lint.IRuleMetadata = {
+        ruleName: "no-null-undefined-union",
+        description: "Disallows union types with both `null` and `undefined` as members.",
+        rationale: Lint.Utils.dedent`
+            A union type that includes both \`null\` and \`undefined\` is either redundant or fragile.
+            Enforcing the choice between the two allows the \`triple-equals\` rule to exist without
+            exceptions, and is essentially a more flexible version of the \`no-null-keyword\` rule.
+        `,
+        optionsDescription: "True if the rule should be enabled.",
+        options: null,
+        optionExamples: [true],
+        type: "functionality",
+        typescriptOnly: true,
+        requiresTypeInfo: true,
+    };
+    /* tslint:enable:object-literal-sort-keys */
+
+    public static FAILURE_STRING = "Union type cannot include both 'null' and 'undefined'.";
+
+    public applyWithProgram(sourceFile: ts.SourceFile, program: ts.Program): Lint.RuleFailure[] {
+        return this.applyWithFunction(sourceFile, walk, undefined, program.getTypeChecker());
+    }
+}
+
+function walk(ctx: Lint.WalkContext<void>, tc: ts.TypeChecker): void {
+    return ts.forEachChild(ctx.sourceFile, function cb(node: ts.Node): void {
+        const type = getType(node, tc);
+        if (type !== undefined && isNullUndefinedUnion(type)) {
+            ctx.addFailureAtNode(node, Rule.FAILURE_STRING);
+        }
+        return ts.forEachChild(node, cb);
+    });
+}
+
+function getType(node: ts.Node, tc: ts.TypeChecker): ts.Type | undefined {
+    // NOTE: This is a comprehensive intersection between `HasType` and has property `name`.
+    // The node name kind must be identifier, or else this rule will throw errors while descending.
+    if (
+        (isVariableDeclaration(node) ||
+            isParameterDeclaration(node) ||
+            isPropertySignature(node) ||
+            isPropertyDeclaration(node) ||
+            isTypeAliasDeclaration(node)) &&
+        node.name.kind === ts.SyntaxKind.Identifier
+    ) {
+        return tc.getTypeAtLocation(node);
+    } else if (isSignatureDeclaration(node)) {
+        const signature = tc.getSignatureFromDeclaration(node);
+        return signature === undefined ? undefined : signature.getReturnType();
+    } else {
+        return undefined;
+    }
+}
+
+function isNullUndefinedUnion(type: ts.Type): boolean {
+    if (isTypeReference(type) && type.typeArguments !== undefined) {
+        return type.typeArguments.some(isNullUndefinedUnion);
+    }
+
+    if (isUnionType(type)) {
+        let hasNull = false;
+        let hasUndefined = false;
+        for (const subType of type.types) {
+            hasNull = hasNull || subType.getFlags() === ts.TypeFlags.Null;
+            hasUndefined = hasUndefined || subType.getFlags() === ts.TypeFlags.Undefined;
+            if (hasNull && hasUndefined) {
+                return true;
+            }
+        }
+    }
+    return false;
+}

src/rules/noUnusedVariableRule.ts

@@ -94,7 +94,7 @@ function parseOptions(options: any[]): Options {
     let ignorePattern: RegExp | undefined;
     for (const o of options) {
         if (typeof o === "object") {
-            // tslint:disable-next-line no-unsafe-any
+            // tslint:disable-next-line no-unsafe-any no-null-undefined-union
             const ignore = o[OPTION_IGNORE_PATTERN] as string | null | undefined;
             if (ignore != undefined) {
                 ignorePattern = new RegExp(ignore);

test/rules/no-null-undefined-union/test.ts.lint

@@ -0,0 +1,55 @@
+[typescript]: >= 2.4.0
+
+interface someInterface {
+    a: number | undefined | null;
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ [0]
+    b: boolean;
+}
+
+const c: string | null | undefined;
+      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ [0]
+
+export type SomeType =
+~~~~~~~~~~~~~~~~~~~~~~
+    | null
+~~~~~~~~~~
+    | undefined
+~~~~~~~~~~~~~~~
+    | boolean;
+~~~~~~~~~~~~~~ [0]
+
+const someFunc = (): string | undefined | null => {}
+                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ [0]
+
+const someFunc = (foo: null | string | undefined, bar: boolean) => {}
+                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ [0]
+
+function someFunc(): number | undefined | null {}
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ [0]
+
+function someFunc(): Promise<number | null | undefined> {} // error
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ [0]
+
+function someFunc(bar: boolean, foo: null | number | undefined) {}
+                                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ [0]
+
+function someFunc() {
+~~~~~~~~~~~~~~~~~~~~~
+    const somePredicate = (): boolean => true;
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+    const someFunc = (): string | null => null;
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+    let foo;
+~~~~~~~~~~~~
+    if (somePredicate()) {
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+        foo = someFunc();
+~~~~~~~~~~~~~~~~~~~~~~~~~
+    }
+~~~~~
+    return foo;
+~~~~~~~~~~~~~~~
+}
+~ [0]
+
+[0]: Union type cannot include both 'null' and 'undefined'.

test/rules/no-null-undefined-union/tsconfig.json

@@ -0,0 +1,19 @@
+{
+  "compilerOptions": {
+    "downlevelIteration": true,
+    "experimentalDecorators": true,
+    "importHelpers": true,
+    "jsx": "react",
+    "lib": [
+        "es5",
+        "es2015",
+        "es2016",
+        "es2017"
+    ],
+    "module": "commonjs",
+    "moduleResolution": "node",
+    "strict": true,
+    "stripInternal": true,
+    "target": "es5"
+  }
+}

test/rules/no-null-undefined-union/tslint.json

@@ -0,0 +1,5 @@
+{
+  "rules": {
+    "no-null-undefined-union": true
+  }
+}