TypeError: X is not a function

"TypeError: X is not a function" from an async function is one of the most common misdiagnoses in Node.js. The pattern: a developer expects getUser(id) to return a user object, but the function was refactored to async and now returns a Promise. Calling getUser(id).name fails because Promise.prototype has no name property — the Promise is being treated as the user object. A related pattern: CommonJS modules that export a function via module.exports = function() {} are imported correctly with const fn = require('./module'). But named exports (exports.fn = function() {}) require destructuring: const { fn } = require('./module'). Mixing these patterns is the most common cause of "is not a function" in projects transitioning from CommonJS to ESM. A third case: in-place mutating list methods in other languages (like Python's list.sort()) have Array equivalents in JavaScript that do return values. But developers from other backgrounds sometimes call the wrong method variant: Array.prototype.sort() modifies in place and returns the original array, while [...arr].sort() is needed for an immutable copy. The diagnostic approach: console.log(typeof X, X) before the failing call reveals whether you have an unresolved Promise, an undefined value, or a module export mismatch.