noNullUndefinedUnionRule.ts

/**
 * @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 { isSignatureDeclaration, isTypeReference, isUnionType, isUnionTypeNode } 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: Lint.Utils.dedent`
            Disallows explicitly declared or implicitly returned 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.
            Optional parameters are not considered to have the type \`undefined\`.
        `,
        optionsDescription: "Not configurable.",
        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, 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 {
    if (isUnionTypeNode(node)) {
        return tc.getTypeAtLocation(node);
    } else if (isSignatureDeclaration(node) && node.type === undefined) {
        // Explicit types should be handled by the first case.
        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;
}