node/tools/eslint/lib/rules/no-unused-expressions.js

/**
 * @fileoverview Flag expressions in statement position that do not side effect
 * @author Michael Ficarra
 * @copyright 2013 Michael Ficarra. All rights reserved.
 */
"use strict";

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

module.exports = function(context) {
    var config = context.options[0] || {},
        allowShortCircuit = config.allowShortCircuit || false,
        allowTernary = config.allowTernary || false;

    /**
     * @param {ASTNode} node - any node
     * @returns {boolean} whether the given node structurally represents a directive
     */
    function looksLikeDirective(node) {
        return node.type === "ExpressionStatement" &&
            node.expression.type === "Literal" && typeof node.expression.value === "string";
    }

    /**
     * @param {Function} predicate - ([a] -> Boolean) the function used to make the determination
     * @param {a[]} list - the input list
     * @returns {a[]} the leading sequence of members in the given list that pass the given predicate
     */
    function takeWhile(predicate, list) {
        for (var i = 0, l = list.length; i < l; ++i) {
            if (!predicate(list[i])) {
                break;
            }
        }
        return [].slice.call(list, 0, i);
    }

    /**
     * @param {ASTNode} node - a Program or BlockStatement node
     * @returns {ASTNode[]} the leading sequence of directive nodes in the given node's body
     */
    function directives(node) {
        return takeWhile(looksLikeDirective, node.body);
    }

    /**
     * @param {ASTNode} node - any node
     * @param {ASTNode[]} ancestors - the given node's ancestors
     * @returns {boolean} whether the given node is considered a directive in its current position
     */
    function isDirective(node, ancestors) {
        var parent = ancestors[ancestors.length - 1],
            grandparent = ancestors[ancestors.length - 2];
        return (parent.type === "Program" || parent.type === "BlockStatement" &&
                (/Function/.test(grandparent.type))) &&
                directives(parent).indexOf(node) >= 0;
    }

    /**
     * Determines whether or not a given node is a valid expression. Recurses on short circuit eval and ternary nodes if enabled by flags.
     * @param {ASTNode} node - any node
     * @returns {boolean} whether the given node is a valid expression
     */
    function isValidExpression(node) {
        if (allowTernary) {
            // Recursive check for ternary and logical expressions
            if (node.type === "ConditionalExpression") {
                return isValidExpression(node.consequent) && isValidExpression(node.alternate);
            }
        }
        if (allowShortCircuit) {
            if (node.type === "LogicalExpression") {
                return isValidExpression(node.right);
            }
        }

        return /^(?:Assignment|Call|New|Update|Yield)Expression$/.test(node.type) ||
            (node.type === "UnaryExpression" && ["delete", "void"].indexOf(node.operator) >= 0);
    }

    return {
        "ExpressionStatement": function(node) {
            if (!isValidExpression(node.expression) && !isDirective(node, context.getAncestors())) {
                context.report(node, "Expected an assignment or function call and instead saw an expression.");
            }
        }
    };

};

module.exports.schema = [
    {
        "type": "object",
        "properties": {
            "allowShortCircuit": {
                "type": "boolean"
            },
            "allowTernary": {
                "type": "boolean"
            }
        },
        "additionalProperties": false
    }
];