noUnnecessaryClassRule.ts

/**
 * @license
 * Copyright 2017 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 * as ts from "typescript";
import * as Lint from "../index";

import {
    getChildOfKind,
    hasModifier,
    isClassDeclaration,
    isConstructorDeclaration,
    isParameterProperty,
} from "tsutils";

interface Options {
    allowConstructorOnly: boolean;
    allowEmptyClass: boolean;
    allowStaticOnly: boolean;
}

const OPTION__ALLOW_CONSTRUCTOR_ONLY = "allow-constructor-only";
const OPTION__ALLOW_EMPTY_CLASS = "allow-empty-class";
const OPTION__ALLOW_STATIC_ONLY = "allow-static-only";

function parseOptions(options: string[]): Options {
    return {
        allowConstructorOnly: options.indexOf(OPTION__ALLOW_CONSTRUCTOR_ONLY) !== -1,
        allowEmptyClass: options.indexOf(OPTION__ALLOW_EMPTY_CLASS) !== -1,
        allowStaticOnly: options.indexOf(OPTION__ALLOW_STATIC_ONLY) !== -1,
    };
}

export class Rule extends Lint.Rules.AbstractRule {
    /* tslint:disable:object-literal-sort-keys */
    public static metadata: Lint.IRuleMetadata = {
        ruleName: "no-unnecessary-class",
        description: Lint.Utils.dedent`
            Disallows classes that are not strictly necessary.`,
        rationale: Lint.Utils.dedent`
            Users who come from a Java-style OO language may wrap
            their utility functions in an extra class, instead of
            putting them at the top level.`,
        optionsDescription: Lint.Utils.dedent`
            Three arguments may be optionally provided:

            * \`"allow-constructor-only"\` ignores classes whose members are constructors.
            * \`"allow-empty-class"\` ignores \`class DemoClass {}\`.
            * \`"allow-static-only"\` ignores classes whose members are static.`,
        options: {
            type: "array",
            items: {
                type: "string",
            },
            minLength: 0,
            maxLength: 3,
        },
        optionExamples: [true, ["allow-empty-class", "allow-constructor-only"]],
        type: "functionality",
        typescriptOnly: false,
    };
    /* tslint:enable:object-literal-sort-keys */

    public static FAILURE_CONSTRUCTOR_ONLY = "Every member of this class is a constructor. Use functions instead.";
    public static FAILURE_STATIC_ONLY = "Every member of this class is static. Use namespaces or plain objects instead.";
    public static FAILURE_EMPTY_CLASS = "This class has no members.";

    public apply(sourceFile: ts.SourceFile): Lint.RuleFailure[] {
        return this.applyWithWalker(
            new NoUnnecessaryClassWalker(sourceFile, this.ruleName, parseOptions(this.ruleArguments)),
        );
    }
}

class NoUnnecessaryClassWalker extends Lint.AbstractWalker<Options> {
    public walk(sourceFile: ts.SourceFile) {
        const checkIfUnnecessaryClass = (node: ts.Node): void => {
            if (isClassDeclaration(node) && !hasExtendsClause(node)) {
                this.checkMembers(node);
            }
            return ts.forEachChild(node, checkIfUnnecessaryClass);
        };
        ts.forEachChild(sourceFile, checkIfUnnecessaryClass);
    }

    private checkMembers(node: ts.ClassDeclaration) {
        if (node.members.length === 0) {
            if (!this.options.allowEmptyClass) {
                this.addFailureAtNode(getChildOfKind(node, ts.SyntaxKind.ClassKeyword)!, Rule.FAILURE_EMPTY_CLASS);
            }
            return;
        }

        const allMembersAreConstructors = node.members.every(isConstructorDeclaration);
        if (
            allMembersAreConstructors &&
            !this.options.allowConstructorOnly &&
            !node.members.some(isConstructorWithShorthandProps)
        ) {
            this.addFailureAtNode(
                getChildOfKind(node, ts.SyntaxKind.ClassKeyword, this.sourceFile)!,
                Rule.FAILURE_CONSTRUCTOR_ONLY,
            );
        }

        if (
            !allMembersAreConstructors &&
            !this.options.allowStaticOnly &&
            !node.members.some(isNonStaticMember)
        ) {
            this.addFailureAtNode(
                getChildOfKind(node, ts.SyntaxKind.ClassKeyword, this.sourceFile)!,
                Rule.FAILURE_STATIC_ONLY,
            );
        }
    }
}

function isNonStaticMember(member: ts.ClassElement): boolean {
    return (
        isConstructorWithShorthandProps(member) ||
        (!isConstructorDeclaration(member) && !hasModifier(member.modifiers, ts.SyntaxKind.StaticKeyword))
    );
}

function hasExtendsClause(declaration: ts.ClassDeclaration): boolean {
    return (
        declaration.heritageClauses !== undefined &&
        declaration.heritageClauses[0].token === ts.SyntaxKind.ExtendsKeyword
    );
}

function isConstructorWithShorthandProps(member: ts.ClassElement): boolean {
    return isConstructorDeclaration(member) && member.parameters.some(isParameterProperty);
}