no-unused-labels.js

/**
 * @fileoverview Rule to disallow unused labels.
 * @author Toru Nagashima
 */

"use strict";

//------------------------------------------------------------------------------
// Requirements
//------------------------------------------------------------------------------

const astUtils = require("./utils/ast-utils");

//------------------------------------------------------------------------------
// Rule Definition
//------------------------------------------------------------------------------

/** @type {import('../shared/types').Rule} */
module.exports = {
    meta: {
        type: "suggestion",

        docs: {
            description: "Disallow unused labels",
            recommended: true,
            url: "https://eslint.org/docs/latest/rules/no-unused-labels"
        },

        schema: [],

        fixable: "code",

        messages: {
            unused: "'{{name}}:' is defined but never used."
        }
    },

    create(context) {
        const sourceCode = context.sourceCode;
        let scopeInfo = null;

        /**
         * Adds a scope info to the stack.
         * @param {ASTNode} node A node to add. This is a LabeledStatement.
         * @returns {void}
         */
        function enterLabeledScope(node) {
            scopeInfo = {
                label: node.label.name,
                used: false,
                upper: scopeInfo
            };
        }

        /**
         * Checks if a `LabeledStatement` node is fixable.
         * For a node to be fixable, there must be no comments between the label and the body.
         * Furthermore, is must be possible to remove the label without turning the body statement into a
         * directive after other fixes are applied.
         * @param {ASTNode} node The node to evaluate.
         * @returns {boolean} Whether or not the node is fixable.
         */
        function isFixable(node) {

            /*
             * Only perform a fix if there are no comments between the label and the body. This will be the case
             * when there is exactly one token/comment (the ":") between the label and the body.
             */
            if (sourceCode.getTokenAfter(node.label, { includeComments: true }) !==
                sourceCode.getTokenBefore(node.body, { includeComments: true })) {
                return false;
            }

            // Looking for the node's deepest ancestor which is not a `LabeledStatement`.
            let ancestor = node.parent;

            while (ancestor.type === "LabeledStatement") {
                ancestor = ancestor.parent;
            }

            if (ancestor.type === "Program" ||
                (ancestor.type === "BlockStatement" && astUtils.isFunction(ancestor.parent))) {
                const { body } = node;

                if (body.type === "ExpressionStatement" &&
                    ((body.expression.type === "Literal" && typeof body.expression.value === "string") ||
                    astUtils.isStaticTemplateLiteral(body.expression))) {
                    return false; // potential directive
                }
            }
            return true;
        }

        /**
         * Removes the top of the stack.
         * At the same time, this reports the label if it's never used.
         * @param {ASTNode} node A node to report. This is a LabeledStatement.
         * @returns {void}
         */
        function exitLabeledScope(node) {
            if (!scopeInfo.used) {
                context.report({
                    node: node.label,
                    messageId: "unused",
                    data: node.label,
                    fix: isFixable(node) ? fixer => fixer.removeRange([node.range[0], node.body.range[0]]) : null
                });
            }

            scopeInfo = scopeInfo.upper;
        }

        /**
         * Marks the label of a given node as used.
         * @param {ASTNode} node A node to mark. This is a BreakStatement or
         *      ContinueStatement.
         * @returns {void}
         */
        function markAsUsed(node) {
            if (!node.label) {
                return;
            }

            const label = node.label.name;
            let info = scopeInfo;

            while (info) {
                if (info.label === label) {
                    info.used = true;
                    break;
                }
                info = info.upper;
            }
        }

        return {
            LabeledStatement: enterLabeledScope,
            "LabeledStatement:exit": exitLabeledScope,
            BreakStatement: markAsUsed,
            ContinueStatement: markAsUsed
        };
    }
};