在if..else语句中未声明变量

I'm just started learning go lang, and I am confused about declaring variables in go lang

for example I've declare req, er inside if...else statement.

if strings.EqualFold(r.Method, "GET") || strings.EqualFold(r.Method, "") {
    req, er := http.NewRequest(r.Method, r.Uri, b)
} else {
    req, er := http.NewRequest(r.Method, r.Uri, b)
}


if er != nil {
    // we couldn't parse the URL.
    return nil, &Error{Err: er}
}

// add headers to the request
req.Host = r.Host
req.Header.Add("User-Agent", r.UserAgent)
req.Header.Add("Content-Type", r.ContentType)
req.Header.Add("Accept", r.Accept)
if r.headers != nil {
    for _, header := range r.headers {
        req.Header.Add(header.name, header.value)
    }
}

But I've got error from terminal

./goreq.go:127: req declared and not used
./goreq.go:127: er declared and not used
./goreq.go:129: req declared and not used
./goreq.go:129: er declared and not used

seems like anything I declared inside If statement is not working... How can I solved it?

Because variables are only defined in the scope in which they are declared:

package main

import "fmt"

func main() {
    a := 1
    fmt.Println(a)
    {
        a := 2
        fmt.Println(a)
    }
    fmt.Println(a)
}

go play

The difference between = and := is that = is just assignment and := is syntax for variable declaration and assigment

This:

a := 1

is equivalent to:

var a int
a = 1

What you probably want is:

var req *http.Request
var er error
if strings.EqualFold(r.Method, "GET") || strings.EqualFold(r.Method, "") {
    req, er = http.NewRequest(r.Method, r.Uri, b)
} else {
    req, er = http.NewRequest(r.Method, r.Uri, b)
}


if er != nil {
    // we couldn't parse the URL.
    return nil, &Error{Err: er}
}

// add headers to the request
req.Host = r.Host
req.Header.Add("User-Agent", r.UserAgent)
req.Header.Add("Content-Type", r.ContentType)
req.Header.Add("Accept", r.Accept)
if r.headers != nil {
    for _, header := range r.headers {
        req.Header.Add(header.name, header.value)
    }
}

This is because you have declared the variables req and er inside the if else condition and are trying to use it outside the scope (the scope is just the if and else inside which they are declared).

You need to declare er and req outside the if else