This error occurs when JavaScript tries to call an async function that hasn't been properly defined or declared. Most commonly, it's caused by missing semicolons between statements, where the parser misinterprets code structure.
This TypeError indicates that you attempted to invoke something as a function, but the value is not actually a function. In the context of async functions, this typically happens when: 1. A function declaration or assignment is incomplete or malformed 2. Missing semicolons cause the parser to misinterpret your code (automatic semicolon insertion failure) 3. The variable name doesn't reference a valid function 4. An async function expression wasn't properly assigned or defined JavaScript's parser becomes confused about where statements begin and end, treating unrelated statements as function calls.
The most common cause is missing a semicolon after require() or variable declarations. When the next line starts with parentheses, JavaScript misinterprets the code:
// ❌ WRONG - Missing semicolon
const fs = require('fs')
(async () => {
console.log('This causes an error');
})();
// ✅ CORRECT - Add semicolon
const fs = require('fs');
(async () => {
console.log('This works fine');
})();Alternatively, place a semicolon before the parentheses:
const fs = require('fs')
;(async () => {
console.log('This also works');
})();Make sure the async function is declared correctly and the variable name is spelled consistently:
// ❌ WRONG - Function not assigned
const asyncFunction {
return 'invalid';
};
// ✅ CORRECT - Proper async function declaration
const asyncFunction = async () => {
return 'valid';
};
// ✅ CORRECT - Async function declaration
async function asyncFunction() {
return 'valid';
}If you're using ES6 modules, ensure the async function is properly exported and imported:
// In module.js
// ❌ WRONG - Forgot export
const asyncFunction = async () => {
return 'data';
};
// ✅ CORRECT - Export the function
export const asyncFunction = async () => {
return 'data';
};
// In main.js
// ✅ Import correctly
import { asyncFunction } from './module.js';
await asyncFunction();Make sure you're not accidentally treating a non-function value as a function:
// ❌ WRONG - asyncFunction is a property, not a method
const obj = {
asyncFunction: 'just a string'
};
await obj.asyncFunction(); // TypeError
// ✅ CORRECT - asyncFunction is an actual function
const obj = {
asyncFunction: async () => {
return 'data';
}
};
await obj.asyncFunction(); // WorksInstead of immediately-invoked async function expressions (IIFE), consider using top-level await in Node.js 14.8+:
// ❌ OLD - IIFE pattern (error-prone)
(async () => {
const data = await fetch('https://api.example.com');
console.log(data);
})();
// ✅ NEW - Top-level await (cleaner, requires "type": "module" in package.json)
const data = await fetch('https://api.example.com');
console.log(data);Make sure your package.json has "type": "module" for ES modules.
Automatic Semicolon Insertion (ASI): JavaScript's ASI rules don't always insert semicolons where you expect. A line starting with [, (, or a few other characters won't trigger ASI from the previous line. This is why explicit semicolons are safer.
CommonJS vs ES Modules: If you're mixing CommonJS (require) and ES modules (import), ensure consistency. The error often occurs when the transition isn't clean.
Async/Await Scope: Remember that await can only be used inside an async function or at the top level of a module (in Node.js 14.8+). Attempting to use await in a non-async context will cause related errors.
Linting: Use ESLint with rules like semi (enforce semicolons) and no-unused-expressions to catch these issues during development.
Error: Listener already called (once event already fired)
EventEmitter listener already called with once()
Error: EACCES: permission denied, open '/root/file.txt'
EACCES: permission denied
Error: Invalid encoding specified (stream encoding not supported)
How to fix Invalid encoding error in Node.js readable streams
Error: EINVAL: invalid argument, open
EINVAL: invalid argument, open
TypeError: readableLength must be a positive integer (stream config)
TypeError: readableLength must be a positive integer in Node.js streams