Go: What is the difference between := and =?

:= — short variable declaration: introduces a new variable and assigns its value. = — assignment: updates an already-declared variable.

Short declarations are the standard way to declare variables in Go functions. The type is inferred from the right-hand side. You need at least one new variable on the left — otherwise the compiler rejects it.

1func example() {
2    name := "Alice"         // new: name
3    age := 30               // new: age
4    name, role := "Bob", "admin"  // OK: role is new
5    name = "Charlie"        // assignment — name exists
6}

Simple assignment requires the variable to already exist in the current scope. Use it to update values, swap variables, or reassign after checks.

1var x int
2x = 42  // assignment
3
4var a, b int
5a, b = b, a  // swap
6
7result, err := doWork()
8if err != nil {
9    err = fmt.Errorf("wrapping: %w", err)  // reassignment
10}

When to use:

• := — first use of a variable, capturing return values, if/for init.
• = — updating, reassigning, swapping existing variables.

Common mistakes:

• Variable shadowing: := inside a nested block creates a new variable that hides the outer one.
• Using := with all existing variables on the left — compiler error.
• := at package level — not allowed; use var.
• Shadowing in for loops: x := creates a new variable each iteration.