--- /dev/null
+/**
+ * @fileoverview Rule to flag comparisons to the value NaN
+ * @author James Allardice
+ */
+
+"use strict";
+
+//------------------------------------------------------------------------------
+// Requirements
+//------------------------------------------------------------------------------
+
+const astUtils = require("./utils/ast-utils");
+
+//------------------------------------------------------------------------------
+// Helpers
+//------------------------------------------------------------------------------
+
+/**
+ * Determines if the given node is a NaN `Identifier` node.
+ * @param {ASTNode|null} node The node to check.
+ * @returns {boolean} `true` if the node is 'NaN' identifier.
+ */
+function isNaNIdentifier(node) {
+ return Boolean(node) && node.type === "Identifier" && node.name === "NaN";
+}
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+
+module.exports = {
+ meta: {
+ type: "problem",
+
+ docs: {
+ description: "require calls to `isNaN()` when checking for `NaN`",
+ category: "Possible Errors",
+ recommended: true,
+ url: "https://eslint.org/docs/rules/use-isnan"
+ },
+
+ schema: [
+ {
+ type: "object",
+ properties: {
+ enforceForSwitchCase: {
+ type: "boolean",
+ default: false
+ },
+ enforceForIndexOf: {
+ type: "boolean",
+ default: false
+ }
+ },
+ additionalProperties: false
+ }
+ ],
+
+ messages: {
+ comparisonWithNaN: "Use the isNaN function to compare with NaN.",
+ switchNaN: "'switch(NaN)' can never match a case clause. Use Number.isNaN instead of the switch.",
+ caseNaN: "'case NaN' can never match. Use Number.isNaN before the switch.",
+ indexOfNaN: "Array prototype method '{{ methodName }}' cannot find NaN."
+ }
+ },
+
+ create(context) {
+
+ const enforceForSwitchCase = context.options[0] && context.options[0].enforceForSwitchCase;
+ const enforceForIndexOf = context.options[0] && context.options[0].enforceForIndexOf;
+
+ /**
+ * Checks the given `BinaryExpression` node for `foo === NaN` and other comparisons.
+ * @param {ASTNode} node The node to check.
+ * @returns {void}
+ */
+ function checkBinaryExpression(node) {
+ if (
+ /^(?:[<>]|[!=]=)=?$/u.test(node.operator) &&
+ (isNaNIdentifier(node.left) || isNaNIdentifier(node.right))
+ ) {
+ context.report({ node, messageId: "comparisonWithNaN" });
+ }
+ }
+
+ /**
+ * Checks the discriminant and all case clauses of the given `SwitchStatement` node for `switch(NaN)` and `case NaN:`
+ * @param {ASTNode} node The node to check.
+ * @returns {void}
+ */
+ function checkSwitchStatement(node) {
+ if (isNaNIdentifier(node.discriminant)) {
+ context.report({ node, messageId: "switchNaN" });
+ }
+
+ for (const switchCase of node.cases) {
+ if (isNaNIdentifier(switchCase.test)) {
+ context.report({ node: switchCase, messageId: "caseNaN" });
+ }
+ }
+ }
+
+ /**
+ * Checks the the given `CallExpression` node for `.indexOf(NaN)` and `.lastIndexOf(NaN)`.
+ * @param {ASTNode} node The node to check.
+ * @returns {void}
+ */
+ function checkCallExpression(node) {
+ const callee = node.callee;
+
+ if (callee.type === "MemberExpression") {
+ const methodName = astUtils.getStaticPropertyName(callee);
+
+ if (
+ (methodName === "indexOf" || methodName === "lastIndexOf") &&
+ node.arguments.length === 1 &&
+ isNaNIdentifier(node.arguments[0])
+ ) {
+ context.report({ node, messageId: "indexOfNaN", data: { methodName } });
+ }
+ }
+ }
+
+ const listeners = {
+ BinaryExpression: checkBinaryExpression
+ };
+
+ if (enforceForSwitchCase) {
+ listeners.SwitchStatement = checkSwitchStatement;
+ }
+
+ if (enforceForIndexOf) {
+ listeners.CallExpression = checkCallExpression;
+ }
+
+ return listeners;
+ }
+};