no-use-before-define.js

/**
 * @fileoverview Rule to flag use of variables before they are defined
 * @author Ilya Volodin
 */

"use strict";

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

const SENTINEL_TYPE = /^(?:(?:Function|Class)(?:Declaration|Expression)|ArrowFunctionExpression|CatchClause|ImportDeclaration|ExportNamedDeclaration)$/u;
const FOR_IN_OF_TYPE = /^For(?:In|Of)Statement$/u;

/**
 * Parses a given value as options.
 * @param {any} options A value to parse.
 * @returns {Object} The parsed options.
 */
function parseOptions(options) {
    let functions = true;
    let classes = true;
    let variables = true;

    if (typeof options === "string") {
        functions = (options !== "nofunc");
    } else if (typeof options === "object" && options !== null) {
        functions = options.functions !== false;
        classes = options.classes !== false;
        variables = options.variables !== false;
    }

    return { functions, classes, variables };
}

/**
 * Checks whether or not a given location is inside of the range of a given node.
 * @param {ASTNode} node An node to check.
 * @param {number} location A location to check.
 * @returns {boolean} `true` if the location is inside of the range of the node.
 */
function isInRange(node, location) {
    return node && node.range[0] <= location && location <= node.range[1];
}

/**
 * Checks whether or not a given location is inside of the range of a class static initializer.
 * @param {ASTNode} node `ClassBody` node to check static initializers.
 * @param {number} location A location to check.
 * @returns {boolean} `true` if the location is inside of a class static initializer.
 */
function isInClassStaticInitializerRange(node, location) {
    return node.body.some(classMember => (
        classMember.type === "PropertyDefinition" &&
        classMember.static &&
        classMember.value &&
        isInRange(classMember.value, location)
    ));
}

/**
 * Checks whether a given scope is the scope of a static class field initializer.
 * @param {eslint-scope.Scope} scope A scope to check.
 * @returns {boolean} `true` if the scope is a class static initializer scope.
 */
function isClassStaticInitializerScope(scope) {
    if (scope.type === "class-field-initializer") {

        // `scope.block` is PropertyDefinition#value node
        const propertyDefinition = scope.block.parent;

        return propertyDefinition.static;
    }

    return false;
}

/**
 * Checks whether a given reference is evaluated in an execution context
 * that isn't the one where the variable it refers to is defined.
 * Execution contexts are:
 * - top-level
 * - functions
 * - class field initializers (implicit functions)
 * Static class field initializers are automatically run during the class definition evaluation,
 * and therefore we'll consider them as a part of the parent execution context.
 * Example:
 *
 *   const x = 1;
 *
 *   x; // returns `false`
 *   () => x; // returns `true`
 *   class C {
 *       field = x; // returns `true`
 *       static field = x; // returns `false`
 *
 *       method() {
 *           x; // returns `true`
 *       }
 *   }
 * @param {eslint-scope.Reference} reference A reference to check.
 * @returns {boolean} `true` if the reference is from a separate execution context.
 */
function isFromSeparateExecutionContext(reference) {
    const variable = reference.resolved;
    let scope = reference.from;

    // Scope#variableScope represents execution context
    while (variable.scope.variableScope !== scope.variableScope) {
        if (isClassStaticInitializerScope(scope.variableScope)) {
            scope = scope.variableScope.upper;
        } else {
            return true;
        }
    }

    return false;
}

/**
 * Checks whether or not a given reference is evaluated during the initialization of its variable.
 *
 * This returns `true` in the following cases:
 *
 *     var a = a
 *     var [a = a] = list
 *     var {a = a} = obj
 *     for (var a in a) {}
 *     for (var a of a) {}
 *     var C = class { [C]; }
 *     var C = class { static foo = C; }
 *     class C extends C {}
 *     class C extends (class { static foo = C; }) {}
 *     class C { [C]; }
 * @param {Reference} reference A reference to check.
 * @returns {boolean} `true` if the reference is evaluated during the initialization.
 */
function isEvaluatedDuringInitialization(reference) {
    if (isFromSeparateExecutionContext(reference)) {

        /*
         * Even if the reference appears in the initializer, it isn't evaluated during the initialization.
         * For example, `const x = () => x;` is valid.
         */
        return false;
    }

    const location = reference.identifier.range[1];
    const definition = reference.resolved.defs[0];

    if (definition.type === "ClassName") {

        // `ClassDeclaration` or `ClassExpression`
        const classDefinition = definition.node;

        return (
            isInRange(classDefinition, location) &&

            /*
             * Class binding is initialized before running static initializers.
             * For example, `class C { static foo = C; }` is valid.
             */
            !isInClassStaticInitializerRange(classDefinition.body, location)
        );
    }

    let node = definition.name.parent;

    while (node) {
        if (node.type === "VariableDeclarator") {
            if (isInRange(node.init, location)) {
                return true;
            }
            if (FOR_IN_OF_TYPE.test(node.parent.parent.type) &&
                isInRange(node.parent.parent.right, location)
            ) {
                return true;
            }
            break;
        } else if (node.type === "AssignmentPattern") {
            if (isInRange(node.right, location)) {
                return true;
            }
        } else if (SENTINEL_TYPE.test(node.type)) {
            break;
        }

        node = node.parent;
    }

    return false;
}

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

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

        docs: {
            description: "disallow the use of variables before they are defined",
            recommended: false,
            url: "https://eslint.org/docs/rules/no-use-before-define"
        },

        schema: [
            {
                oneOf: [
                    {
                        enum: ["nofunc"]
                    },
                    {
                        type: "object",
                        properties: {
                            functions: { type: "boolean" },
                            classes: { type: "boolean" },
                            variables: { type: "boolean" }
                        },
                        additionalProperties: false
                    }
                ]
            }
        ],

        messages: {
            usedBeforeDefined: "'{{name}}' was used before it was defined."
        }
    },

    create(context) {
        const options = parseOptions(context.options[0]);

        /**
         * Determines whether a given reference should be checked.
         *
         * Returns `false` if the reference is:
         * - initialization's (e.g., `let a = 1`).
         * - referring to an undefined variable (i.e., if it's an unresolved reference).
         * - referring to a variable that is defined, but not in the given source code
         *   (e.g., global environment variable or `arguments` in functions).
         * - allowed by options.
         * @param {eslint-scope.Reference} reference The reference
         * @returns {boolean} `true` if the reference should be checked
         */
        function shouldCheck(reference) {
            if (reference.init) {
                return false;
            }

            const variable = reference.resolved;

            if (!variable || variable.defs.length === 0) {
                return false;
            }

            const definitionType = variable.defs[0].type;

            if (!options.functions && definitionType === "FunctionName") {
                return false;
            }

            if (
                (
                    !options.variables && definitionType === "Variable" ||
                    !options.classes && definitionType === "ClassName"
                ) &&

                // don't skip checking the reference if it's in the same execution context, because of TDZ
                isFromSeparateExecutionContext(reference)
            ) {
                return false;
            }

            return true;
        }

        /**
         * Finds and validates all references in a given scope and its child scopes.
         * @param {eslint-scope.Scope} scope The scope object.
         * @returns {void}
         */
        function checkReferencesInScope(scope) {
            scope.references.filter(shouldCheck).forEach(reference => {
                const variable = reference.resolved;
                const definitionIdentifier = variable.defs[0].name;

                if (
                    reference.identifier.range[1] < definitionIdentifier.range[1] ||
                    isEvaluatedDuringInitialization(reference)
                ) {
                    context.report({
                        node: reference.identifier,
                        messageId: "usedBeforeDefined",
                        data: reference.identifier
                    });
                }
            });

            scope.childScopes.forEach(checkReferencesInScope);
        }

        return {
            Program() {
                checkReferencesInScope(context.getScope());
            }
        };
    }
};