var works at both package and function scope with explicit typing, while := is a shorthand only valid inside functions that infers the type from the assigned value.
var x int — works at package or function scope, allows explicit type, sets zero value if no initializer
x := 5 — only valid inside functions, infers type, requires an initial value
:= can declare multiple variables at once: x, err := someFunc()
:= allows re-declaration if at least one variable on the left is new
var is preferred for package-level variables and when zero value initialization is intentional
Common pitfall: := inside an inner scope shadows the outer variable rather than updating it. This causes subtle bugs especially with the error variable. Always check if := is creating a new variable or shadowing an existing one.
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