How to fix TypeError: The 'path' argument must be of type string. Received type number in Node.js

Node.jsBEGINNERMEDIUMUpdated Dec 22, 2025· 193 views

This error occurs when you pass an argument of the wrong type to a Node.js function that expects a string. It's commonly triggered when file system methods receive a number, undefined, null, or object instead of a string path.

Quick fix

Fixes ERR_INVALID_ARG_TYPE

TypeError: The 'path' argument must be of type string. Received type number
    at validatePath (node:internal/fs/utils:168:13)
    at Object.readFile (node:fs/promises:32:18)
    at /path/to/your/file.js:15:22
  1. Locate the exact line causing the error
  2. Verify the argument type before passing it
  3. Ensure the path variable is properly initialized
  4. Fix accidental type conversions
node.js — error
TypeError: The 'path' argument must be of type string. Received type number
    at validatePath (node:internal/fs/utils:168:13)
    at Object.readFile (node:fs/promises:32:18)
    at /path/to/your/file.js:15:22
TypeError: The 'path' argument must be of type string. Received type number

What this error means

The ERR_INVALID_ARG_TYPE error is a TypeError that Node.js throws when a function receives an argument of an unexpected type. This is a strict type validation error that helps catch programmer mistakes early. In the case of the 'path' argument, many Node.js APIs (fs.readFile, fs.writeFile, path.join, etc.) require a string, Buffer, or URL object for file paths. When you accidentally pass a number, boolean, undefined, null, or plain object, Node.js immediately throws this error to prevent silent failures or undefined behavior. This error is usually a result of incorrect variable types in your code—either from accessing undefined properties, type conversions gone wrong, or misunderstanding what a function expects. The error message clearly indicates both what type was expected and what type was actually received, making it straightforward to debug.

Typical symptoms

  • Node.js process crashes with TypeError during file system operations
  • Error appears with message mentioning 'path' argument and a specific type name
  • Functions like fs.readFile, fs.writeFile, or path operations fail unexpectedly
  • The error happens when using variables that should contain file paths
  • Code works with hardcoded strings but fails with variables
  • Application crashes with ERR_INVALID_ARG_TYPE in logs

Common causes

  1. Passing a number instead of a string to a function expecting a path
  2. Variable containing the path is undefined due to missing initialization
  3. Accessing an object property that does not exist, resulting in undefined
  4. Incorrect type conversion or casting of values
  5. Destructuring or accessing optional properties without null checks
  6. Using parameters from environment variables or config files without validation
  7. Type mismatch between function arguments in TypeScript compilation
  8. Passing null or a plain object where a string is expected

How to fix "TypeError: The 'path' argument must be of type string. Received type number"

1Locate the exact line causing the error

Check the stack trace to identify which function call is failing. The error message will show the file and line number:

bash
TypeError: The 'path' argument must be of type string. Received type number
    at validatePath (node:internal/fs/utils:168:13)
    at Object.readFile (node:fs/promises:32:18)
    at /path/to/your/file.js:15:22

Look at line 15 in your file to see what's being passed to readFile.

2Verify the argument type before passing it

Add console.log or use a debugger to check what type the variable actually is:

javascript
import { readFile } from 'fs/promises';

let filePath = getUserPath(); // Could be undefined or wrong type
console.log('filePath type:', typeof filePath);
console.log('filePath value:', filePath);

// This might fail if filePath is not a string
const data = await readFile(filePath, 'utf-8');

Use typeof or console.log to inspect the actual value before it reaches the function.

3Ensure the path variable is properly initialized

Make sure your path variable is defined and contains a string before use:

javascript
// ❌ BAD - filePath might be undefined
const filePath = config.path;
await readFile(filePath);

// ✓ GOOD - Provide a default or check for undefined
const filePath = config.path ?? './default.txt';
if (typeof filePath !== 'string') {
  throw new Error('Path must be a string');
}
await readFile(filePath);

Use optional chaining and nullish coalescing operators to handle missing values safely.

4Fix accidental type conversions

If you intended to use a string but received a different type, convert it explicitly:

javascript
// ❌ BAD - Passing a number accidentally
const fileIndex = 5;
const data = await readFile(fileIndex); // Fails: number is not a string

// ✓ GOOD - Convert to string explicitly
const filePath = String(fileIndex); // Converts 5 to '5'
const data = await readFile(filePath);

// ✓ GOOD - Or use string interpolation
const filePath = `file_${fileIndex}.txt`;
const data = await readFile(filePath);

// ✓ GOOD - For arrays, use proper path joining
import { join } from 'path';
const dirName = 'data';
const fileName = 'file.txt';
const filePath = join(dirName, fileName); // Returns 'data/file.txt'
const data = await readFile(filePath);

Use explicit type conversion or the path.join utility instead of string concatenation.

5Add type checking for destructured properties

When destructuring objects, check that properties exist before using them:

javascript
// ❌ BAD - Destructuring could fail silently
const { filePath } = config;
await readFile(filePath); // filePath is undefined if config lacks this property

// ✓ GOOD - Provide defaults in destructuring
const { filePath = './default.txt' } = config;
if (typeof filePath !== 'string') {
  throw new Error('filePath must be a string');
}
await readFile(filePath);

// ✓ GOOD - Explicit validation
const { filePath } = config;
if (!filePath || typeof filePath !== 'string') {
  throw new Error('Invalid filePath: must be a non-empty string');
}
await readFile(filePath);

Always validate destructured values before using them in functions with strict type requirements.

6Use TypeScript to catch type errors at compile time

If using TypeScript, define proper types for your variables:

typescript
import { readFile } from 'fs/promises';

interface Config {
  filePath: string; // Explicitly require string
}

async function loadData(config: Config): Promise<string> {
  // TypeScript will error if filePath is not a string
  const data = await readFile(config.filePath, 'utf-8');
  return data;
}

// ✓ This compiles fine
await loadData({ filePath: './data.txt' });

// ❌ TypeScript error: Type 'number' is not assignable to type 'string'
await loadData({ filePath: 123 });

TypeScript's type system prevents these errors before runtime.

Advanced notes

Understanding Argument Type Validation in Node.js

Node.js strictly validates argument types for built-in functions to prevent silent failures. Unlike JavaScript where type coercion often occurs automatically, Node.js APIs are strict about types. For example:

javascript
// JavaScript allows loose type coercion
console.log('5' + 3); // '53' - concatenation

// But Node.js fs methods don't allow this coercion
const fs = require('fs');
fs.readFileSync(123); // ❌ Throws ERR_INVALID_ARG_TYPE, doesn't coerce
fs.readFileSync('123'); // ✓ Works - expects string path

This strictness helps catch bugs early and makes behavior predictable.

Validation with Zod or Runtime Type Checking

For applications handling user input or configuration, use schema validation libraries:

javascript
import { z } from 'zod';
import { readFile } from 'fs/promises';

const ConfigSchema = z.object({
  filePath: z.string().min(1, 'filePath must not be empty'),
});

async function loadConfig(raw: unknown) {
  try {
    const config = ConfigSchema.parse(raw);
    const data = await readFile(config.filePath, 'utf-8');
    return data;
  } catch (err) {
    if (err instanceof z.ZodError) {
      console.error('Invalid config:', err.errors);
    } else {
      throw err;
    }
  }
}

Zod validates input at runtime and provides detailed error messages.

Common Functions with Type Restrictions

These Node.js functions strictly validate the 'path' argument:
- fs.readFile(path, ...) - expects string, Buffer, or URL
- fs.writeFile(path, data, ...) - expects string, Buffer, or URL
- fs.access(path, ...) - expects string, Buffer, or URL
- path.join(...paths) - expects all arguments to be strings
- path.resolve(...paths) - expects all arguments to be strings
- fs.stat(path, ...) - expects string, Buffer, or URL

Debugging Type Issues

Use Object.prototype.toString.call() for accurate type detection:

javascript
function getType(value) {
  return Object.prototype.toString.call(value).slice(8, -1);
}

console.log(getType('hello')); // 'String'
console.log(getType(123)); // 'Number'
console.log(getType(undefined)); // 'Undefined'
console.log(getType(null)); // 'Null'
console.log(getType({})); // 'Object'
console.log(getType([])); // 'Array'

This is more reliable than typeof for complex types.

Buffer and URL as Valid Path Types

Some Node.js functions accept Buffer or URL in addition to strings:

javascript
import { readFile } from 'fs/promises';
import { fileURLToPath } from 'url';

// ✓ String path
await readFile('./file.txt');

// ✓ Buffer path
await readFile(Buffer.from('./file.txt'));

// ✓ URL object
const fileUrl = new URL('file:///home/user/file.txt');
await readFile(fileUrl);

// ✓ File URL to path conversion
const filePath = fileURLToPath(import.meta.url);
await readFile(filePath);

Check function documentation to see which types are accepted for each API.

Frequently asked questions

What does "ERR_INVALID_ARG_TYPE" mean?
js throws when a function receives an argument of an unexpected type. This is a strict type validation error that helps catch programmer mistakes early. ) require a string, Buffer, or URL object for file paths.
How do I fix "ERR_INVALID_ARG_TYPE"?
This error occurs when you pass an argument of the wrong type to a Node.js function that expects a string. It's commonly triggered when file system methods receive a number, undefined, null, or object instead of a string path.
What causes "ERR_INVALID_ARG_TYPE"?
Passing a number instead of a string to a function expecting a path Variable containing the path is undefined due to missing initialization Accessing an object property that does not exist, resulting in undefined
Is the "ERR_INVALID_ARG_TYPE" error serious?
It is a medium-severity error — more of a blocker than a risk. The steps above resolve it.

Official resources & further reading