Programming Fundamentals › Programming Basics

Type Coercion

Implicit conversion by the language, like "5" + 1 in JavaScript.

Also known as: implicit conversion, automatic type conversion

Type coercion is the language converting a value from one type to another for you, implicitly, so that an operation can go ahead. It’s the opposite of explicit type conversion, where you call int() or Number() yourself.

JavaScript is famous for it:

"5" + 1       // "51"    number turned into a string
"5" - 1       // 4       string turned into a number
"5" * "2"     // 10
true + 1      // 2
[] + {}       // "[object Object]"
null == 0     // false, but null >= 0 is true

The same + does different things depending on the types, and the rules are hard to remember.

Languages differ

• JavaScript, PHP, SQL (some dialects): lots of implicit coercion.
• Python, Go, Rust, Java (mostly): refuse most mixes, and raise a TypeError instead:

"5" + 1       # TypeError: can only concatenate str to str

Python does coerce some numbers: 1 + 2.5 is 3.5, and True + 1 is 2.

Why it bites

• Form and URL values arrive as strings, and arithmetic silently does the wrong thing.
• Loose equality == converts types first, so 0 == "" is true (== vs ===).
• Conditions treat many values as true or false (truthy and falsy).
• Database comparisons between a string column and a number can skip indexes.

Defenses

• Convert explicitly at the boundary (int(value), Number(value)) and validate the result.
• Use strict equality (===).
• Use a typed language or TypeScript to catch mixes early (static vs dynamic typing).
• When a result looks absurd ("11" instead of 2), suspect coercion.