javascript filter implementation

There are many built-in functions in Javascript, filter() is one of them. Its function is to filter out elements that meet the criteria from an array and return a new array.

The syntax of filter() is as follows:

array.filter(function(currentValue, index, arr), thisValue)

Parameter description:

• currentValue: The current element being processed in the array
• index: The subscript of the current element in the array
• arr: the array being processed
• thisValue (optional): use this value as this when the object is executed as a function (that is, the function execution context)

The filter() method accepts a function as a parameter, which accepts three parameters: the current element, the index of the current element, and the entire array. The function must return a Boolean value indicating whether the current element meets the conditions. If true is returned, the element will be retained in the new array, otherwise it will be filtered out.

Let's look at an example, assuming there is an array storing integers from 1 to 10.

const numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

Now we want to filter out all elements greater than or equal to 5. This can be achieved through the filter() method:

const result = numbers.filter(function(number) {
  return number >= 5;
});

console.log(result); // 输出 [5, 6, 7, 8, 9, 10]

In the above code, the filter() function receives a function whose return value is true or false. In this function, true is returned if the current element is greater than or equal to 5. Otherwise, if the current element is less than 5, false is returned. Finally, the elements that meet the criteria, that is, elements greater than or equal to 5, are retained and returned in the form of a new array.

In addition to defining a function, you can also use the arrow function abbreviation:

const result = numbers.filter(number => number >= 5);

console.log(result); // 输出 [5, 6, 7, 8, 9, 10]

The filter() method can also be used on objects in an array. Suppose you have an array that stores information about multiple people.

const people = [
  { name: 'Lucas', age: 25 },
  { name: 'Emma', age: 21 },
  { name: 'Tom', age: 30 },
  { name: 'Jane', age: 18 },
];

Now we want to filter out people who are 25 or older. This can be achieved through the filter() method:

const result = people.filter(function(person) {
  return person.age >= 25;
});

console.log(result); // 输出 [{ name: 'Lucas', age: 25 }, { name: 'Tom', age: 30 }]

Similarly, we can also use the arrow function abbreviation:

const result = people.filter(person => person.age >= 25);

console.log(result); // 输出 [{ name: 'Lucas', age: 25 }, { name: 'Tom', age: 30 }]

In actual development, the filter() method is often used to filter data that meets conditions , such as in search, filtering, classification and other scenarios. It is simple and easy to use, and can quickly filter array elements. It can be said to be one of the indispensable methods in development.

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