HTML forms can send an
HTTP
request declaratively. But forms can also prepare an HTTP request to send via JavaScript, for example via
XMLHttpRequest
. This article explores such approaches.
With progressive web apps, single page apps, and framework based apps, it's common to use HTML 表单 to send data without loading a new document when response data is received. Let's first talk about why this requires a different approach.
Standard HTML form submission, as described in the previous article, loads the URL where the data was sent, which means the browser window navigates with a full page load. Avoiding a full page load can provide a smoother experience by avoiding network lag, and possible visual issues like flickering.
Many modern UIs only use HTML forms to collect input from the user, and not for data submission. When the user tries to send the data, the application takes control and transmits the data asynchronously in the background, updating only the parts of the UI that require changes.
Sending arbitrary data asynchronously is generally called AJAX , which stands for "Asynchronous JavaScript And XML" .
XMLHttpRequest
(XHR) DOM object can build HTTP requests, send them, and retrieve their results. Historically,
XMLHttpRequest
was designed to fetch and send
XML
as an exchange format, which has since been superseded by
JSON
. But neither XML nor JSON fit into form data request encoding. Form data (
application/x-www-form-urlencoded
) is made of URL-encoded lists of key/value pairs. For transmitting binary data, the HTTP request is reshaped into
multipart/form-data
.
注意： 抓取 API is often used in place of XHR these days — it is a modern, updated version of XHR, which works in a similar fashion but has some advantages. Most of the XHR code you'll see in this article could be swapped out for Fetch.
If you control the front-end (the code that's executed in the browser) and the back-end (the code which is executed on the server), you can send JSON/XML and process them however you want.
But if you want to use a third party service, you need to send the data in the format the services require.
So how should we send such data? The different techniques you'll require are done below.
There are 3 ways to send form data:
XMLHttpRequest
manually.
FormData
对象。
FormData
bound to a
<form>
元素。
Let's look at them in detail.
XMLHttpRequest
is the safest and most reliable way to make HTTP requests. To send form data with
XMLHttpRequest
, prepare the data by URL-encoding it, and obey the specifics of form data requests.
Let's look at an example:
<button>Click Me!</button>
And now the JavaScript:
const btn = document.querySelector('button');
function sendData( data ) {
console.log( 'Sending data' );
const XHR = new XMLHttpRequest();
let urlEncodedData = "",
urlEncodedDataPairs = [],
name;
// Turn the data object into an array of URL-encoded key/value pairs.
for( name in data ) {
urlEncodedDataPairs.push( encodeURIComponent( name ) + '=' + encodeURIComponent( data[name] ) );
}
// Combine the pairs into a single string and replace all %-encoded spaces to
// the '+' character; matches the behavior of browser form submissions.
urlEncodedData = urlEncodedDataPairs.join( '&' ).replace( /%20/g, '+' );
// Define what happens on successful data submission
XHR.addEventListener( 'load', function(event) {
alert( 'Yeah! Data sent and response loaded.' );
} );
// Define what happens in case of error
XHR.addEventListener( 'error', function(event) {
alert( 'Oops! Something went wrong.' );
} );
// Set up our request
XHR.open( 'POST', 'https://example.com/cors.php' );
// Add the required HTTP header for form data POST requests
XHR.setRequestHeader( 'Content-Type', 'application/x-www-form-urlencoded' );
// Finally, send our data.
XHR.send( urlEncodedData );
}
btn.addEventListener( 'click', function() {
sendData( {test:'ok'} );
} )
Here's the live result:
注意：
This use of
XMLHttpRequest
is subject to the
same-origin policy
if you want to send data to a third party web site. For cross-origin requests, you'll need
CORS and HTTP access control
.
Building an HTTP request by hand can be overwhelming. Fortunately, the
XMLHttpRequest specification
provides a newer, simpler way to handle form data requests with the
FormData
对象。
FormData
object can be used to build form data for transmission, or to get the data within a form element to manage how it's sent. Note that
FormData
objects are "write only", which means you can change them, but not retrieve their contents.
Using this object is detailed in Using FormData Objects , but here are two examples:
<button>Click Me!</button>
You should be familiar with that HTML sample. Now for the JavaScript:
const btn = document.querySelector('button');
function sendData( data ) {
const XHR = new XMLHttpRequest(),
FD = new FormData();
// Push our data into our FormData object
for( name in data ) {
FD.append( name, data[ name ] );
}
// Define what happens on successful data submission
XHR.addEventListener( 'load', function( event ) {
alert( 'Yeah! Data sent and response loaded.' );
} );
// Define what happens in case of error
XHR.addEventListener(' error', function( event ) {
alert( 'Oops! Something went wrong.' );
} );
// Set up our request
XHR.open( 'POST', 'https://example.com/cors.php' );
// Send our FormData object; HTTP headers are set automatically
XHR.send( FD );
}
btn.addEventListener( 'click', function()
{ sendData( {test:'ok'} );
} )
Here's the live result:
You can also bind a
FormData
object to an
<form>
element. This creates a
FormData
object that represents the data contained in the form.
The HTML is typical:
<form id="myForm">
<label for="myName">Send me your name:</label>
<input id="myName" name="name" value="John">
<input type="submit" value="Send Me!">
</form>
But JavaScript takes over the form:
window.addEventListener( "load", function () {
function sendData() {
const XHR = new XMLHttpRequest();
// Bind the FormData object and the form element
const FD = new FormData( form );
// Define what happens on successful data submission
XHR.addEventListener( "load", function(event) {
alert( event.target.responseText );
} );
// Define what happens in case of error
XHR.addEventListener( "error", function( event ) {
alert( 'Oops! Something went wrong.' );
} );
// Set up our request
XHR.open( "POST", "https://example.com/cors.php" );
// The data sent is what the user provided in the form
XHR.send( FD );
}
// Access the form element...
const form = document.getElementById( "myForm" );
// ...and take over its submit event.
form.addEventListener( "submit", function ( event ) {
event.preventDefault();
sendData();
} );
} );
Here's the live result:
You can even get more involved with the process by using the form's
元素
property to get a list of all of the data elements in the form and manually manage them one at a time. To learn more about that, see the example in
Accessing the element list's contents
in
HTMLFormElement.elements
.
If you use a
FormData
object with a form that includes
<input type="file">
widgets, the data will be processed automatically. But to send binary data by hand, there's extra work to do.
There are many sources for binary data, including
FileReader
,
Canvas
，和
WebRTC
. Unfortunately, some legacy browsers can't access binary data or require complicated workarounds. To learn more about the
FileReader
API, see
使用来自 Web 应用程序的文件
.
The least complicated way of sending binary data is by using
FormData
's
append()
method, demonstrated above. If you have to do it by hand, it's trickier.
In the following example, we use the
FileReader
API to access binary data and then build the multi-part form data request by hand:
<form id="theForm">
<p>
<label for="theText">text data:</label>
<input id="theText" name="myText" value="Some text data" type="text">
</p>
<p>
<label for="theFile">file data:</label>
<input id="theFile" name="myFile" type="file">
</p>
<button>Send Me!</button>
</form>
As you see, the HTML is a standard
<form>
. There's nothing magical going on. The "magic" is in the JavaScript:
// Because we want to access DOM nodes,
// we initialize our script at page load.
window.addEventListener( 'load', function () {
// These variables are used to store the form data
const text = document.getElementById( "theText" );
const file = {
dom : document.getElementById( "theFile" ),
binary : null
};
// Use the FileReader API to access file content
const reader = new FileReader();
// Because FileReader is asynchronous, store its
// result when it finishes to read the file
reader.addEventListener( "load", function () {
file.binary = reader.result;
} );
// At page load, if a file is already selected, read it.
if( file.dom.files[0] ) {
reader.readAsBinaryString( file.dom.files[0] );
}
// If not, read the file once the user selects it.
file.dom.addEventListener( "change", function () {
if( reader.readyState === FileReader.LOADING ) {
reader.abort();
}
reader.readAsBinaryString( file.dom.files[0] );
} );
// sendData is our main function
function sendData() {
// If there is a selected file, wait it is read
// If there is not, delay the execution of the function
if( !file.binary && file.dom.files.length > 0 ) {
setTimeout( sendData, 10 );
return;
}
// To construct our multipart form data request,
// We need an XMLHttpRequest instance
const XHR = new XMLHttpRequest();
// We need a separator to define each part of the request
const boundary = "blob";
// Store our body request in a string.
let data = "";
// So, if the user has selected a file
if ( file.dom.files[0] ) {
// Start a new part in our body's request
data += "--" + boundary + "\r\n";
// Describe it as form data
data += 'content-disposition: form-data; '
// Define the name of the form data
+ 'name="' + file.dom.name + '"; '
// Provide the real name of the file
+ 'filename="' + file.dom.files[0].name + '"\r\n';
// And the MIME type of the file
data += 'Content-Type: ' + file.dom.files[0].type + '\r\n';
// There's a blank line between the metadata and the data
data += '\r\n';
// Append the binary data to our body's request
data += file.binary + '\r\n';
}
// Text data is simpler
// Start a new part in our body's request
data += "--" + boundary + "\r\n";
// Say it's form data, and name it
data += 'content-disposition: form-data; name="' + text.name + '"\r\n';
// There's a blank line between the metadata and the data
data += '\r\n';
// Append the text data to our body's request
data += text.value + "\r\n";
// Once we are done, "close" the body's request
data += "--" + boundary + "--";
// Define what happens on successful data submission
XHR.addEventListener( 'load', function( event ) {
alert( 'Yeah! Data sent and response loaded.' );
} );
// Define what happens in case of error
XHR.addEventListener( 'error', function( event ) {
alert( 'Oops! Something went wrong.' );
} );
// Set up our request
XHR.open( 'POST', 'https://example.com/cors.php' );
// Add the required HTTP header to handle a multipart form data POST request
XHR.setRequestHeader( 'Content-Type','multipart/form-data; boundary=' + boundary );
// And finally, send our data.
XHR.send( data );
}
// Access our form...
const form = document.getElementById( "theForm" );
// ...to take over the submit event
form.addEventListener( 'submit', function ( event ) {
event.preventDefault();
sendData();
} );
} );
Here's the live result:
Depending on the browser and the type of data you are dealing with, sending form data through JavaScript can be easy or difficult. The
FormData
object is generally the answer, and you can use a
polyfill
for it on legacy browsers.
最后修改： , 由 MDN 贡献者