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| --- | ||
| title: Working with URL encoded form values | ||
| date: 2023-09-19 | ||
| categories: | ||
| - articles | ||
| --- | ||
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| In the book, we discussed how to send and received multipart form data. That is, | ||
| forms containing both text fields and file uploads, typically using the content | ||
| type as `multipart/form-data`. When we are working with only text fields, it is | ||
| sufficient to instead use the content type, `application/x-www-form-urlencoded` | ||
| and send the data as a _URL encoded set of key value pairs_. For example, as | ||
| described by the [MDN article](https://developer.mozilla.org/en-US/docs/Learn/Forms/Sending_and_retrieving_form_data#on_the_client_side_defining_how_to_send_the_data), | ||
| `say=Hi&to=Rheo` will be the request payload for a form with two text fields, `say` and `to` with the values, | ||
| `Hi` and `Rheo` respectively. | ||
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| In this blog post, we will write a client and a server to send and process form data. | ||
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| Let's get started! | ||
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| - [Prerequisites](#prerequisites) | ||
| - [Reading URL encoded form data in a Go server](#reading-url-encoded-form-data-in-a-go-server) | ||
| - [Using a browser as the client](#using-a-browser-as-the-client) | ||
| - [Sending URL encoded form data from a Go client](#sending-url-encoded-form-data-from-a-go-client) | ||
| - [Learn more](#learn-more) | ||
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| ## Prerequisites | ||
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| - An installation of Go | ||
| - A tool to edit your code. Any text editor you have will work fine | ||
| - A command terminal to run the `go` commands from | ||
|
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| ## Reading URL encoded form data in a Go server | ||
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| Reading URL encoded form data in a server's handler function involves two key steps: | ||
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| 1. Call [`ParseForm()`](https://pkg.go.dev/net/http#Request.ParseForm) method of the request object | ||
| 1. Then, retrieve the values of the fields using [`FormValue()`](https://pkg.go.dev/net/http#Request.FormValue) | ||
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| Example code snippet: | ||
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| ```go | ||
| func handleForm(w http.ResponseWriter, req *http.Request) { | ||
| // this will look for application/x-www-form-urlencoded | ||
| // content type | ||
| err := req.ParseForm() | ||
| if err != nil { | ||
| // if there was an error here, we return the error | ||
| // as response along with a 400 http response code | ||
| http.Error(w, err.Error(), http.StatusBadRequest) | ||
| return | ||
| } | ||
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| // once the form has been parsed correctly | ||
| // we can obtain the form fields using | ||
| // req.FormValue passing the field name as the key | ||
| // see ./post-method.html for the field names | ||
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| // it's worth noting here that FormValue() will return | ||
| // the first value if there are multiple values specified | ||
| // in the request | ||
| // if you want to access the multiple values specified, | ||
| // use req.Form directly (see https://pkg.go.dev/net/http#Request) | ||
| say := req.FormValue("say") | ||
| to := req.FormValue("to") | ||
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| // do something with the values read | ||
| .. | ||
| } | ||
| ``` | ||
| You can find the complete code for the form handler in | ||
| [form_handler.go](https://github.com/practicalgo/form-url-encoded-demo). | ||
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| To run the application: | ||
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| ``` | ||
| go build | ||
| ./go-form-url-encoded.md # or .\go-form-url-encoded.exe | ||
| ``` | ||
|
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| ## Using a browser as the client | ||
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| First, let's make a request to the server from a browser by visiting http://localhost:8000. | ||
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| [Here](https://youtu.be/UX-ZWlq36EQ) is a video demonstration of using the application from a | ||
| browser. | ||
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| Key points to note from the above video are: | ||
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| 1. The "raw" request data (payload) is: `say=Hi&to=Rheo` | ||
| 2. The content-type header is set as, `Content-Type: application/x-www-form-urlencoded` | ||
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| Both are automatically done for us when we click on the "Send my greetings" button. | ||
|
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| ## Sending URL encoded form data from a Go client | ||
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| Now, let's say you want to write a test for the `handleForm()` handler function. | ||
| You will need to send the relevant data as URL encoded form fields. Here is | ||
| a code snippet showing you how: | ||
|
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| ```go | ||
| func TestHandleForm(t *testing.T) { | ||
| w := httptest.NewRecorder() | ||
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| // set the key value pairs | ||
| formBody := url.Values{ | ||
| "say": []string{"hi"}, // we only have a single element, but these need to be a slice | ||
| "to": []string{"rheo"}, // same as above | ||
| } | ||
| dataReader := formBody.Encode() // this perform the URL encoding and returns us a string | ||
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| req := httptest.NewRequest( | ||
| http.MethodPost, | ||
| "http://localhost", | ||
| strings.NewReader(dataReader), // we need a io.Reader | ||
| ) | ||
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| // set the content-type header | ||
| req.Header.Add("Content-Type", "application/x-www-form-urlencoded") | ||
| handleForm(w, req) | ||
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| // rest of the test | ||
| } | ||
| ``` | ||
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| You can find the complete code for the form handler test function in | ||
| [form_handler_test.go](https://github.com/practicalgo/form-url-encoded-demo). | ||
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| Outside of test functions, you can use the same approach with `http.NewRequest` or | ||
| `http.NewRequestWithContext`. | ||
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| Key references: | ||
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| - [`url.Values`](https://pkg.go.dev/net/url#Values) and [`Encode()`](https://pkg.go.dev/net/url#Values.Encode) | ||
| method | ||
| - [`strings.NewReader()`](https://pkg.go.dev/strings#NewReader) | ||
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| ## Learn more | ||
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| - [Sending and retrieving form data](https://developer.mozilla.org/en-US/docs/Learn/Forms/Sending_and_retrieving_form_data) | ||
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|
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