What is an error in Go language?

Overview

The Go language has an in-built type error that is used for error handling. error simplifies identifying which function/s or code returns an error, and why the error has occurred. To use error, you must first import the errors package, as shown below:

import (
"errors"
)

An error type can return as nil or not. If it is nil, then there is no error.

Example

The following example will help you understand the error type better. In the example below, the user is first asked to input their age into an online screening system for job applications. After acquiring their age, we use a program to check whether it’s invalid, underage, or valid. If their age is invalid or underage, the required error is returned to indicate so. If not, the error returned is nil.

package main
import (
  "fmt"
  "errors"
)
func check(age int)(string, error){
  if age < 0{ //code for invalide age of employee
    return "",errors.New("Invalid age")
  } else if age < 20{ //code for invalide age of employee
    return "",errors.New("Under age")
  } else { //if the age is above 20
    return "Can apply!", nil
  }
  
}
func main() {
    //take input from user
    fmt.Println("Enter Your Age: ")
    var age int
    fmt.Scanln(&age)
    validity, err := check(age)
    if err != nil {
      fmt.Println(err)
    } else {
      fmt.Println(validity)
    }
}

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Apart from New(), there are three more functions in the error package:

Name	Prototype	Description
`As()`	`func As(err error, target interface{}) bool`	Looks for error in an error chain. Upon success, sets `target` to the error and returns `true`.
`Is()`	`func Is(err, target error) bool`	If the `target` matches the error, then the function returns `true`.
`Unwrap()`	`func Unwrap(err error) error`	Returns an error if the `Unwrap()` call to an error returns an error. Otherwise, it returns `nil`.

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