--- /dev/null
+/**
+ * @fileoverview Ensures that the results of typeof are compared against a valid string
+ * @author Ian Christian Myers
+ */
+"use strict";
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+
+module.exports = {
+ meta: {
+ type: "problem",
+
+ docs: {
+ description: "enforce comparing `typeof` expressions against valid strings",
+ category: "Possible Errors",
+ recommended: true,
+ url: "https://eslint.org/docs/rules/valid-typeof"
+ },
+
+ schema: [
+ {
+ type: "object",
+ properties: {
+ requireStringLiterals: {
+ type: "boolean",
+ default: false
+ }
+ },
+ additionalProperties: false
+ }
+ ],
+ messages: {
+ invalidValue: "Invalid typeof comparison value.",
+ notString: "Typeof comparisons should be to string literals."
+ }
+ },
+
+ create(context) {
+
+ const VALID_TYPES = ["symbol", "undefined", "object", "boolean", "number", "string", "function", "bigint"],
+ OPERATORS = ["==", "===", "!=", "!=="];
+
+ const requireStringLiterals = context.options[0] && context.options[0].requireStringLiterals;
+
+ /**
+ * Determines whether a node is a typeof expression.
+ * @param {ASTNode} node The node
+ * @returns {boolean} `true` if the node is a typeof expression
+ */
+ function isTypeofExpression(node) {
+ return node.type === "UnaryExpression" && node.operator === "typeof";
+ }
+
+ //--------------------------------------------------------------------------
+ // Public
+ //--------------------------------------------------------------------------
+
+ return {
+
+ UnaryExpression(node) {
+ if (isTypeofExpression(node)) {
+ const parent = context.getAncestors().pop();
+
+ if (parent.type === "BinaryExpression" && OPERATORS.indexOf(parent.operator) !== -1) {
+ const sibling = parent.left === node ? parent.right : parent.left;
+
+ if (sibling.type === "Literal" || sibling.type === "TemplateLiteral" && !sibling.expressions.length) {
+ const value = sibling.type === "Literal" ? sibling.value : sibling.quasis[0].value.cooked;
+
+ if (VALID_TYPES.indexOf(value) === -1) {
+ context.report({ node: sibling, messageId: "invalidValue" });
+ }
+ } else if (requireStringLiterals && !isTypeofExpression(sibling)) {
+ context.report({ node: sibling, messageId: "notString" });
+ }
+ }
+ }
+ }
+
+ };
+
+ }
+};