goNull / UndefinedMedium

Interface nil comparison bug with typed nil

Time: 0:00

BugFind the bug and fix the code

What it should do

Check if custom error pointer is nil and return boolean status. Example: checkCustomError(false) -> true

What happens instead

nil error pointer compared to error interface returns false instead of true

package main

import "fmt"

type CustomErr struct {
	Msg string
}

func (e *CustomErr) Error() string {
	return e.Msg
}

func getError(hasErr bool) error {
	var err *CustomErr = nil
	if hasErr {
		err = &CustomErr{Msg: "failed"}
	}
	return err
}

func checkCustomError(hasErr bool) bool {
	err := getError(hasErr)
	// Interface has non-nil type (*CustomErr), so err != nil is true!
	return err == nil
}

func main() {
	fmt.Println(checkCustomError(false))
}

Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).

⏱️0:00

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This bug belongs to the null and undefined errors pattern

Cannot read properties of undefined, NoneType has no attribute, NaN appearing from nowhere.

Learn the null and undefined errors pattern Step through your own code

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package main import "fmt" type CustomErr struct { Msg string } func (e *CustomErr) Error() string { return e.Msg } func getError(hasErr bool) error { var err *CustomErr = nil if hasErr { err = &CustomErr{Msg: "failed"} } return err } func checkCustomError(hasErr bool) bool { err := getError(hasErr) // Interface has non-nil type (*CustomErr), so err != nil is true! return err == nil } func main() { fmt.Println(checkCustomError(false)) }

Interface nil comparison bug with typed nil

What it should do

What happens instead

More null and undefined errors to practise

Related errors

Interface nil comparison bug with typed nil

What it should do

What happens instead

More null and undefined errors to practise

Related errors