Python + JavaScript

TypeError: What It Means and How to Fix It

Error reference

TypeError

Language

Python + JavaScript

Severity

High

When it happens

A TypeError is raised when an operation is applied to a value of the wrong type.

Potential fixes

Convert values to the correct type before using them (e.g. int(), str() in Python). In JavaScript, guard against null and undefined with optional chaining (?.) or explicit checks before calling methods.

Related errors

ValueError AttributeError

Deep dive

A TypeError is raised when an operation is applied to a value of the wrong type. In Python it commonly appears when mixing incompatible types in an expression. In JavaScript it appears when you try to call a method on null, undefined, or a non-function.

Why it happens

•Mixing strings and numbers in arithmetic without explicit conversion (Python).
•Calling a method on null or undefined (JavaScript).
•Passing the wrong number of arguments to a function.
•Treating a non-iterable value as if it were iterable.

Potential fixes

Convert values to the correct type before using them (e.g. int(), str() in Python). In JavaScript, guard against null and undefined with optional chaining (?.) or explicit checks before calling methods.

Examples

Python

Code that triggers the error

age = "25"
result = age + 5
print(result)

Error output

Traceback (most recent call last):
  File "app.py", line 2, in <module>
    result = age + 5
TypeError: can only concatenate str (not "int") to str

Fixed code

age = "25"
result = int(age) + 5
print(result)  # 30

JavaScript

Code that triggers the error

const value = null;
console.log(value.toUpperCase());

Error output

TypeError: Cannot read properties of null (reading 'toUpperCase')

Fixed code

const value = null;
if (value !== null && value !== undefined) {
  console.log(value.toUpperCase());
}

// Or with optional chaining:
console.log(value?.toUpperCase());

Practice in English

How would you explain a TypeError to a fellow dev? Choose the right phrase:

"I hit an error...

Ready to practice your English at work?

Lingua-e has interactive exercises built around real developer conversations: standups, code reviews, retrospectives, and more. Practice until it comes naturally.

Try Lingua-e for free