Type errors: when a number is secretly a string
Totals that concatenate, sorts that put 10 before 9, comparisons that refuse to run.
What is a type error?
A value is not the type the code assumes. Python usually raises immediately; JavaScript usually converts silently and gives a wrong answer, which is worse because nothing tells you anything went wrong.
Why it happens
Data crossing a boundary loses its type. Form fields, JSON, query strings and CSV files all arrive as text. JSON object keys are always strings even when they look like numbers. JavaScript's + means addition or concatenation depending on its operands, and its default sort compares elements as strings.
How to recognise it
- A total is a string of digits stuck together rather than a sum.
- Sorting puts 10 before 9 — string ordering, not numeric.
- KeyError on a key you can see in the printed dict (it is "1", not 1).
- In Python 3, / always returns a float, so it can never be an index.
Error guides caused by this pattern
All error guides →In-depth guides for common exceptions and runtime errors triggered by type errors, with working fixes and real-world examples:
How to fix it
Convert once, at the boundary where data enters, rather than scattering int() and Number() through your logic. Use === over == in JavaScript so type mismatches fail loudly. When a value surprises you, print its type before you print its value.
Breaks vs Works Example
Breaks
// JavaScript: string concatenation instead of addition
const total = inputField.value + 10; // '20' + 10 = '2010'Works
// JavaScript: explicit numeric conversion at input boundary
const total = Number(inputField.value) + 10; // 20 + 10 = 30💡 Form values, URL query params, and JSON strings must be parsed at boundaries.
Practise type errors
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