The filter method of JavaScript is used to create a new array containing all elements tested by the callback function. 1. Basic usage: filter traverses each element of the array, and when the callback returns true, the element is retained, such as filtering numbers greater than 25; 2. Filtering object array: You can filter through object properties, such as selecting users with age greater than or equal to 18; 3. Multi-condition filtering: You can use logical operators to combine multiple conditions in the callback, such as meeting age greater than 20 and the name contains the letter "a"; 4. Dynamic filtering: Use the input box to realize the real-time search function, and dynamically update the filter results based on user input to ensure that upper and lower case insensitive.

Using JavaScript's filter method to filter array elements is actually a very practical and concise operation. It allows you to pick out members that meet the criteria from an array, generate a new array without changing the original array. The key is to write the judgment conditions, that is, the logic in the callback function.

Basic usage: How to write the simplest filter

The filter method will iterate over each element in the array and execute the callback function you provide for each element. If the callback returns true, the element will be retained and placed in the new array.

For example:

 const numbers = [10, 20, 30, 40, 50];
const filtered = numbers.filter(num => num > 25);

The above code will leave numbers greater than 25, and the result is [30, 40, 50] . This is one of the most common uses of filter, suitable for filtering data that meets a certain numerical criteria.

Filtering object array: Filter data by attributes

Filter is also very useful when you are dealing with an array of objects. For example, you have a set of user data that you want to filter based on age, status or other fields.

For example:

 const users = [
  { name: 'Alice', age: 25 },
  { name: 'Bob', age: 17 },
  { name: 'Charlie', age: 30 }
];

const adults = users.filter(user => user.age >= 18);

This will allow adults to be picked out. This kind of scenario is particularly common when displaying data on the front end, such as only displaying activated accounts, unfinished tasks, etc.

Multi-condition filtering: Combination judgment is more flexible

Sometimes one condition alone is not enough, and multiple conditions need to be judged together. At this time, you can add logical judgment to the callback function.

For example, we want to find someone older than 20 and whose name contains the letter "a":

 const results = users.filter(user => user.age > 20 && user.name.includes('a'));

Or you can write it more clearly:

 users.filter(user => {
  return user.age > 20 && user.name.toLowerCase().includes('a');
});

The small detail here is toLowerCase() to avoid case affecting the matching result. This multi-condition writing method is very practical in search and filtering functions.

Dynamic filtering with input box: Make the page move

filter is not only a static data processing tool, but also can perform real-time filtering with user input. For example, when making a search box, the list will be automatically updated as soon as the user types.

The HTML structure is roughly like this:

 <input type="text" id="search">
<ul id="list"></ul>

The JS part can be written like this:

 const names = ['Apple', 'Banana', 'Cherry', 'Date'];
const input = document.getElementById('search');
const list = document.getElementById('list');

input.addEventListener('input', () => {
  const keyword = input.value.toLowerCase();
  const filtered = names.filter(name => name.toLowerCase().includes(keyword));

  list.innerHTML = filtered.map(name => `<li>${name}</li>`).join(&#39;&#39;);
});

This allows for a simple but effective search filtering function. Note that toLowerCase() has been used twice to ensure that the case does not affect the search results.

Basically that's it. Although the filter method is simple, it can solve many practical problems if used properly. The key is to understand its mechanism and how to write the correct judgment conditions.

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