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How to use both map and filter on array using JavaScript?

PHPzforward: 2023-08-28 20:29:05875browse

如何使用 JavaScript 在数组上同时使用映射和过滤器？

The filter() method is used to filter the values in the array, and the map() method is used to map new values to another array based on each value of the old array.

Sometimes, we need to use the filter() and map() methods at the same time. For example, we want to filter out all positive numbers from an array and map their logarithmic values to a new array

Before starting this tutorial, let's take a look at the introduction of the filter() and map() methods. In this tutorial, we will learn how to use both mapping and filtering methods on arrays using JavaScript.

Syntax of array.filter() method

Users can use the JavaScript filter() method according to the following syntax.

array.filter((element, index, self) => {
   // write a condition to filter values.
   
   // based on the filtering condition, return a boolean value to include in the filtered array.
})

In the above syntax, we pass the callback function as a parameter of the filter() method.

Syntax of array.map() method

Users can use the JavaScript map() method according to the following syntax.

array.map((element, index, self) => {
   // perform some action on the element of the array
   
   // return element for a new array
})

In the above syntax, we need to return the array elements from the callback function of the map() method.

parameter

element – When iterating over an array using the filter() method, it is the current element of the array.
index – It is the index of the element in the array.
self – It is itself an array.

Use array.map() and array.filter() methods simultaneously

This section will teach us to use the filter() and map() methods together on a single array.

grammar

Users can use the map() and filter() methods together according to the following syntax.

let logarithmic_values = array.filter((element, index, self) => {
   
   // filtering condition
})
.map((element, index, self) => {
   // return value from map method
})

In the above syntax, we first use the filter() method on the array, and then use the map() method.

Example 1

In the example below, the array contains positive and negative numbers. We take the array as a reference and call the filter() method on the array to filter all positive values in the array. In the callback function of the filter() method, we return true if the number is greater than zero; otherwise an error.

After that, we use the map() method and return the logarithm of each filtered element. The user can see that the logarithmic_values array contains only six values because we have removed all negative values in the filtered array.

<html>
<body>
   <h3>Using the <i> array.map() and array.filter() </i> methods together in JavaScript</h3>
   <div id = "output"> </div>
   <script>
      let output = document.getElementById('output');
      let array = [10, 20, -2, -4, 32, -6, -7, -8, 10, 11, -20]
      let logarithmic_values = array.filter((element, index, self) => {
         if (element > 0) {
            return true;
         }
         return false;
      })
      .map((element, index, self) => {
         return Math.log(element);
      })
      output.innerHTML += "The original array values are " + array + "<br/>";
      output.innerHTML += "The logarithmic_values are " + logarithmic_values + "<br/>";
   </script>
</body>
</html>

Example 2

In the following example, we create an array of objects, each object in the array contains employee ID, years of service, and salary.

After that, we use the filter() method to filter all employees with more than 3 years of experience. Next, we use the map() method to increase the salaries of all employees by 50% and store the new salaries in the new_salaries array.

In the output, the user can initially observe the total salary after the increment.

<html>
<body>
   <h2>Using the <i> array.map() and array.filter() </i> methods together in JavaScript </h2>
   <div id = "output"> </div>
   <script>
      let output = document.getElementById('output');
      
      // Creating the array of objects
      let array = [{ emp_id: "01", experience: 3, salary: 50000 },
      { emp_id: "02", experience: 7, salary: 30000 },
      { emp_id: "03", experience: 6, salary: 20000 },
      { emp_id: "04", experience: 5, salary: 10000 },
      { emp_id: "05", experience: 3, salary: 5000 },
      { emp_id: "06", experience: 2, salary: 40000 },
      { emp_id: "07", experience: 1.5, salary: 60000 },
      { emp_id: "08", experience: 2, salary: 70000 }]
      
      // use the forEach() loop method to find the total initial salary
      let total_initial_salary = 0;
      array.forEach((employee) => {
         total_initial_salary += employee.salary;
      })
      
      // filter all employees with experience greater than 3 years.
      let new_salaries = array.filter((element) => {
         if (element.experience > 3) {
            return true;
         }
         return false;
      })
      .map((element) => {
         
         // increase the salary of all employees by 50%, who have experienced greater than 3 years
         return element.salary * 0.5;
      })
      
      // find the sum of new salaries
      let new_salary = total_initial_salary;
      let total_increased_salary = new_salaries.forEach((salary) => {
         new_salary += salary
      })
      output.innerHTML += "The initial total salaries of all employees is " + total_initial_salary + "<br/>";
      output.innerHTML += "The total salaries of all employees after increasing salaries for some employees is " + new_salary + "<br/>";
   </script>
</body>
</html>

Users learned how to use the filter() and map() methods together through various examples. In the first example, we use the filter() and map() methods with an array of numbers. In the second example, we also learned how to use the filter() and map() methods with an array of objects.

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