🧪 Go – Type Assertion Explained with Syntax, Examples & Use Cases (2025 Guide)

🧲 Introduction – What Is Type Assertion in Go?

In Go, type assertion is used to extract the underlying concrete value from an interface{} variable. It allows you to convert an interface type to its actual type—safely or unsafely. Type assertion is widely used in generic interfaces, JSON decoding, and polymorphic behavior.

🎯 In this section, you’ll learn:

• How to use type assertion syntax in Go
• Perform safe vs unsafe type assertions
• Use type switches for multiple type handling
• Real-world examples and common pitfalls

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✅ Basic Type Assertion Syntax

var i interface{} = "GoLang"
str := i.(string)
fmt.Println(str)

📤 Output:

GoLang

✅ Here, i.(string) asserts that the underlying type stored in i is a string.

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⚠️ Unsafe Assertion – Runtime Panic

var i interface{} = 123
str := i.(string)  // ❌ This will panic

📛 Output:

panic: interface conversion: interface {} is int, not string

❗ Use only when you’re absolutely sure of the type.

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✅ Safe Assertion Using Comma-Ok Idiom

var i interface{} = "Hello"
str, ok := i.(string)

if ok {
    fmt.Println("String value:", str)
} else {
    fmt.Println("Not a string")
}

📤 Output:

String value: Hello

✅ Avoids panic—ok returns true if assertion is valid.

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🔁 Type Switch – Handle Multiple Types Safely

var val interface{} = 3.14

switch v := val.(type) {
case int:
    fmt.Println("Integer:", v)
case float64:
    fmt.Println("Float:", v)
case string:
    fmt.Println("String:", v)
default:
    fmt.Println("Unknown type")
}

📤 Output:

Float: 3.14

✅ The type switch simplifies checking multiple possible types.

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🧱 Real-World Use – JSON Decoding

data := map[string]interface{}{
    "age": 30,
    "name": "John",
}

age, ok := data["age"].(int)
name, ok2 := data["name"].(string)

fmt.Println("Age:", age, "Name:", name)

✅ Commonly used when decoding interface{} maps returned from JSON.

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⚖️ Type Assertion vs Type Conversion

Feature	Type Assertion	Type Conversion
Works on	interface{} types	Any compatible types
Purpose	Extract real type	Convert value types
Syntax	val.(T)	T(val)
Fails if mismatched?	✅ Runtime panic	✅ Compile-time error

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🧠 Best Practices

• ✅ Always use comma-ok idiom unless you’re sure of the type
• ✅ Use type switch for multi-type logic
• ❌ Avoid overusing interface{} unless necessary
• ✅ Keep interfaces small and focused for easy type assertions

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📌 Summary – Recap & Next Steps

Type assertion is a vital tool in Go for extracting the real type from interface variables. It enables safe type conversion and is widely used in generic programming and decoding dynamic data.

🔍 Key Takeaways:

• Use i.(T) to assert type
• Use i.(T), ok to avoid panics
• Use type switch to handle multiple types
• Validate types when decoding from interface{}

⚙️ Next: Explore Go Reflection, Interface Composition, or Generics with type constraints.

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❓ FAQs – Go Type Assertion

❓ What happens if type assertion fails?
❌ It causes a runtime panic, unless handled with the comma-ok idiom.

❓ How is type assertion different from type conversion?
✅ Type assertion extracts the underlying value from an interface{}. Conversion changes one value type to another.

❓ When should I use a type switch?
✅ Use a type switch when checking for multiple possible types.

❓ Can I assert to a struct type?
✅ Yes, as long as the interface holds a value of that struct type.

❓ Is type assertion available for all interfaces?
✅ Yes, but it only works if the underlying type matches the asserted type.

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