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node_modules/eslint/lib/rules/no-unused-expressions.js

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+/**
+ * @fileoverview Flag expressions in statement position that do not side effect
+ * @author Michael Ficarra
+ */
+"use strict";
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+
+module.exports = {
+    meta: {
+        type: "suggestion",
+
+        docs: {
+            description: "disallow unused expressions",
+            category: "Best Practices",
+            recommended: false,
+            url: "https://eslint.org/docs/rules/no-unused-expressions"
+        },
+
+        schema: [
+            {
+                type: "object",
+                properties: {
+                    allowShortCircuit: {
+                        type: "boolean",
+                        default: false
+                    },
+                    allowTernary: {
+                        type: "boolean",
+                        default: false
+                    },
+                    allowTaggedTemplates: {
+                        type: "boolean",
+                        default: false
+                    }
+                },
+                additionalProperties: false
+            }
+        ]
+    },
+
+    create(context) {
+        const config = context.options[0] || {},
+            allowShortCircuit = config.allowShortCircuit || false,
+            allowTernary = config.allowTernary || false,
+            allowTaggedTemplates = config.allowTaggedTemplates || false;
+
+        // eslint-disable-next-line jsdoc/require-description
+        /**
+         * @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";
+        }
+
+        // eslint-disable-next-line jsdoc/require-description
+        /**
+         * @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 (let i = 0; i < list.length; ++i) {
+                if (!predicate(list[i])) {
+                    return list.slice(0, i);
+                }
+            }
+            return list.slice();
+        }
+
+        // eslint-disable-next-line jsdoc/require-description
+        /**
+         * @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);
+        }
+
+        // eslint-disable-next-line jsdoc/require-description
+        /**
+         * @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) {
+            const parent = ancestors[ancestors.length - 1],
+                grandparent = ancestors[ancestors.length - 2];
+
+            return (parent.type === "Program" || parent.type === "BlockStatement" &&
+                    (/Function/u.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);
+                }
+            }
+
+            if (allowTaggedTemplates && node.type === "TaggedTemplateExpression") {
+                return true;
+            }
+
+            return /^(?:Assignment|Call|New|Update|Yield|Await)Expression$/u.test(node.type) ||
+                (node.type === "UnaryExpression" && ["delete", "void"].indexOf(node.operator) >= 0);
+        }
+
+        return {
+            ExpressionStatement(node) {
+                if (!isValidExpression(node.expression) && !isDirective(node, context.getAncestors())) {
+                    context.report({ node, message: "Expected an assignment or function call and instead saw an expression." });
+                }
+            }
+        };
+
+    }
+};