Type Assertions

A type assertion checks whether an interface value holds a particular concrete type or satisfies another interface.

The safe form number, ok := value.(int) returns a value and a boolean. When the assertion fails, ok is false and number has the zero value of int.

The one-result form number := value.(int) panics if the assertion fails. Use the two-result form when encountering user input, decoded values or other uncertain dynamic types.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import "fmt"

func main() {
	var data any = "hello"
	text, ok := data.(string)
	fmt.Println("String:", text, ok)
	number, valid := data.(int)
	fmt.Println("Integer:", number, valid)
}

Output

String: hello true
Integer: 0 false

data holds a string dynamically, so the string assertion succeeds with true. The int assertion fails safely and returns integer zero plus false.

• Asserting an incorrect type without comma-ok panics.
• A type assertion works on interface expressions, not arbitrary concrete variables.

Put an integer into a variable of type any, then perform successful int and unsuccessful string assertions.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced Type Assertions in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.