Type Assertions & Type Switches

Type assertions

A type assertion extracts the concrete value stored inside an interface:

The two-value (safe) form never panics:

Always prefer the two-value form unless a wrong type is a programming error you want to crash on.

Type switches

A type switch tests an interface value against multiple concrete types in one construct:

Inside each case, x is already typed — no further assertion needed.

The any type

any is an alias for interface{} (introduced in Go 1.18). Functions that accept arbitrary values use any:

Checking error types

Type assertions are the mechanism behind errors.As:

You can also use a type switch directly on errors:

Interface satisfaction check

Use a blank identifier assertion as a compile-time check that a type implements an interface:

Common pitfalls

• Asserting a nil interface panics: var w io.Writer; w.(*os.File) — always check for nil first
• fmt.Errorf("...: %w", err) wraps errors — use errors.As rather than type asserting the raw error to unwrap correctly
• Asserting to an interface (not a concrete type) is valid and often useful for capability checks