javascript multi-select box to select several

JavaScript is a widely used scripting language that can be used to create interactive web pages and applications. Among them, the multi-select box is a commonly used form element in web development, allowing the user to select one or more options. In this article, we will explore how to select several checkboxes using JavaScript.

1. Basic usage of HTML multi-select box

To define a multi-select box in HTML, you need to use the tag and set the corresponding attribute values: name, value and checked. Among them, the name attribute is used to define the name of the multi-select box; the value attribute is used to specify the value of the option; the checked attribute is used to set the default selected item.

For example, the following code creates a multi-select box named "fruits" that includes three options: apples, bananas, and oranges, with apples being the default selected item.

<form>
  <input type="checkbox" name="fruits" value="apple" checked>苹果<br>
  <input type="checkbox" name="fruits" value="banana">香蕉<br>
  <input type="checkbox" name="fruits" value="orange">橙子
</form>

Users can check or uncheck options by clicking on the checkbox. When the user submits the form, the selected options are submitted to the server as form data.

2. JavaScript implements multi-select box selection.

Although the multi-select box function provided by HTML is very convenient, sometimes we need to operate the selected options on the client, such as Get the number of selected items or loop through selected options. At this time, we can use JavaScript to get the number of selected multi-select boxes.

1. Get the number of selected multiple selection boxes

To get the number of selected multiple selection boxes, we can use the document.querySelectorAll() method to get all selected multiple selections box elements and count their number. This method returns a list of all elements matching the specified CSS selector. For example, the following code obtains the selected number in the multi-select box named "fruits":

const checkboxes = document.querySelectorAll('input[name="fruits"]:checked');
const checkedCount = checkboxes.length;
console.log(`选中了 ${checkedCount} 个选项。`);

In this code, the first line first obtains the number of selected items in the multi-select box named "fruits" through the document.querySelectorAll() method. All selected elements in the selection box and store them in a NodeList object. Note that the CSS selector 'input[name="fruits"]:checked' is used to match selected checkbox elements. Among them, the name attribute specifies the name of the multi-select box to be selected, and the :checked pseudo-class can be used to select the selected element.

The second line calculates the number of selected multi-select boxes through checkboxes.length. Finally, use the console.log() method to output the results.

2. Processing the selected multi-select box

To process the selected multi-select box, we can use code similar to the above to get the selected multi-select box element , and then use a loop to traverse each selected element and perform the corresponding operation therein.

For example, the following code iterates through the selected elements in the multi-select box and adds their values ​​to a string:

const checkboxes = document.querySelectorAll('input[name="fruits"]:checked');
let values = '';
checkboxes.forEach(checkbox => {
  values += checkbox.value + ',';
});
console.log(`选中的选项是：${values}`);

The code first obtains the list of selected multi-select box elements, Then use the forEach() method to loop through each selected element. In the loop body, add the value of the current element to the values ​​variable and output the result at the end.

3. Summary

In this article, we introduced the basic usage of HTML multi-select boxes, and used JavaScript to implement the function of selecting several multi-select boxes. By getting the number of selected multi-select boxes and processing the selected multi-select box elements, we extend the functionality of the multi-select box and provide a basis for implementing complex form interactions. I hope this article will be helpful to web developers learning and using JavaScript to handle multi-select boxes.

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