no-eval.js

/**
 * @fileoverview Rule to flag use of eval() statement
 * @author Nicholas C. Zakas
 */

"use strict";

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

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

//------------------------------------------------------------------------------
// Helpers
//------------------------------------------------------------------------------

const candidatesOfGlobalObject = Object.freeze([
    "global",
    "window",
    "globalThis"
]);

/**
 * Checks a given node is a MemberExpression node which has the specified name's
 * property.
 * @param {ASTNode} node A node to check.
 * @param {string} name A name to check.
 * @returns {boolean} `true` if the node is a MemberExpression node which has
 *      the specified name's property
 */
function isMember(node, name) {
    return astUtils.isSpecificMemberAccess(node, null, name);
}

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

module.exports = {
    meta: {
        type: "suggestion",

        docs: {
            description: "disallow the use of `eval()`",
            category: "Best Practices",
            recommended: false,
            url: "https://eslint.org/docs/rules/no-eval"
        },

        schema: [
            {
                type: "object",
                properties: {
                    allowIndirect: { type: "boolean", default: false }
                },
                additionalProperties: false
            }
        ],

        messages: {
            unexpected: "eval can be harmful."
        }
    },

    create(context) {
        const allowIndirect = Boolean(
            context.options[0] &&
            context.options[0].allowIndirect
        );
        const sourceCode = context.getSourceCode();
        let funcInfo = null;

        /**
         * Pushs a variable scope (Program or Function) information to the stack.
         *
         * This is used in order to check whether or not `this` binding is a
         * reference to the global object.
         * @param {ASTNode} node A node of the scope. This is one of Program,
         *      FunctionDeclaration, FunctionExpression, and ArrowFunctionExpression.
         * @returns {void}
         */
        function enterVarScope(node) {
            const strict = context.getScope().isStrict;

            funcInfo = {
                upper: funcInfo,
                node,
                strict,
                defaultThis: false,
                initialized: strict
            };
        }

        /**
         * Pops a variable scope from the stack.
         * @returns {void}
         */
        function exitVarScope() {
            funcInfo = funcInfo.upper;
        }

        /**
         * Reports a given node.
         *
         * `node` is `Identifier` or `MemberExpression`.
         * The parent of `node` might be `CallExpression`.
         *
         * The location of the report is always `eval` `Identifier` (or possibly
         * `Literal`). The type of the report is `CallExpression` if the parent is
         * `CallExpression`. Otherwise, it's the given node type.
         * @param {ASTNode} node A node to report.
         * @returns {void}
         */
        function report(node) {
            const parent = node.parent;
            const locationNode = node.type === "MemberExpression"
                ? node.property
                : node;

            const reportNode = parent.type === "CallExpression" && parent.callee === node
                ? parent
                : node;

            context.report({
                node: reportNode,
                loc: locationNode.loc,
                messageId: "unexpected"
            });
        }

        /**
         * Reports accesses of `eval` via the global object.
         * @param {eslint-scope.Scope} globalScope The global scope.
         * @returns {void}
         */
        function reportAccessingEvalViaGlobalObject(globalScope) {
            for (let i = 0; i < candidatesOfGlobalObject.length; ++i) {
                const name = candidatesOfGlobalObject[i];
                const variable = astUtils.getVariableByName(globalScope, name);

                if (!variable) {
                    continue;
                }

                const references = variable.references;

                for (let j = 0; j < references.length; ++j) {
                    const identifier = references[j].identifier;
                    let node = identifier.parent;

                    // To detect code like `window.window.eval`.
                    while (isMember(node, name)) {
                        node = node.parent;
                    }

                    // Reports.
                    if (isMember(node, "eval")) {
                        report(node);
                    }
                }
            }
        }

        /**
         * Reports all accesses of `eval` (excludes direct calls to eval).
         * @param {eslint-scope.Scope} globalScope The global scope.
         * @returns {void}
         */
        function reportAccessingEval(globalScope) {
            const variable = astUtils.getVariableByName(globalScope, "eval");

            if (!variable) {
                return;
            }

            const references = variable.references;

            for (let i = 0; i < references.length; ++i) {
                const reference = references[i];
                const id = reference.identifier;

                if (id.name === "eval" && !astUtils.isCallee(id)) {

                    // Is accessing to eval (excludes direct calls to eval)
                    report(id);
                }
            }
        }

        if (allowIndirect) {

            // Checks only direct calls to eval. It's simple!
            return {
                "CallExpression:exit"(node) {
                    const callee = node.callee;

                    /*
                     * Optional call (`eval?.("code")`) is not direct eval.
                     * The direct eval is only step 6.a.vi of https://tc39.es/ecma262/#sec-function-calls-runtime-semantics-evaluation
                     * But the optional call is https://tc39.es/ecma262/#sec-optional-chaining-chain-evaluation
                     */
                    if (!node.optional && astUtils.isSpecificId(callee, "eval")) {
                        report(callee);
                    }
                }
            };
        }

        return {
            "CallExpression:exit"(node) {
                const callee = node.callee;

                if (astUtils.isSpecificId(callee, "eval")) {
                    report(callee);
                }
            },

            Program(node) {
                const scope = context.getScope(),
                    features = context.parserOptions.ecmaFeatures || {},
                    strict =
                        scope.isStrict ||
                        node.sourceType === "module" ||
                        (features.globalReturn && scope.childScopes[0].isStrict);

                funcInfo = {
                    upper: null,
                    node,
                    strict,
                    defaultThis: true,
                    initialized: true
                };
            },

            "Program:exit"() {
                const globalScope = context.getScope();

                exitVarScope();
                reportAccessingEval(globalScope);
                reportAccessingEvalViaGlobalObject(globalScope);
            },

            FunctionDeclaration: enterVarScope,
            "FunctionDeclaration:exit": exitVarScope,
            FunctionExpression: enterVarScope,
            "FunctionExpression:exit": exitVarScope,
            ArrowFunctionExpression: enterVarScope,
            "ArrowFunctionExpression:exit": exitVarScope,

            ThisExpression(node) {
                if (!isMember(node.parent, "eval")) {
                    return;
                }

                /*
                 * `this.eval` is found.
                 * Checks whether or not the value of `this` is the global object.
                 */
                if (!funcInfo.initialized) {
                    funcInfo.initialized = true;
                    funcInfo.defaultThis = astUtils.isDefaultThisBinding(
                        funcInfo.node,
                        sourceCode
                    );
                }

                if (!funcInfo.strict && funcInfo.defaultThis) {

                    // `this.eval` is possible built-in `eval`.
                    report(node.parent);
                }
            }
        };

    }
};