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JavaScript Objects and Functions

Being a fourth generation programming paradigm language, the semantics and structure of Objects and Functions in JavaScript are quite closely tied together.

The JavaScript code to make an object from a function

In JavaScript, you can create an object from a function by using a constructor function. A constructor function is a special type of function that is used to create objects
. Here's an example of how to create an object from a function:
javascriptfunction Person(name, age) { this.name = name; this.age = age;}
const person1 = new Person("John", 30);const person2 = new Person("Jane", 25);
console.log(person1); // { name: "John", age: 30 }console.log(person2); // { name: "Jane", age: 25 }
In the above example, the Person function is used as a constructor function to create two objects person1 and person2. The new keyword is used to create a new instance of the Person function, which is assigned to the variables person1 and person2. The this keyword is used to assign values to the object's properties based on the values passed to the function
.
Alternatively, you can also create an object using an object literal syntax. Here's an example:
javascriptconst person = { name: "John", age: 30, greet: function() { console.log("Hello!"); }};
console.log(person); // { name: "John", age: 30, greet: [Function: greet] }
In the above example, an object person is created using an object literal syntax. The object has two properties name and age, and a method greet that logs a message to the console
.

What is the difference between using object literal and constructor function to create an object in JavaScript

The main difference between using object literal and constructor function to create an object in JavaScript is that an object created by the object literal is a singleton, which means when a change is made to the object, it affects that object across the entire script. Whereas, if an object is created using a constructor function and a change is made to it, that change won't affect the object throughout the script
.
Another difference is that with the constructor function notation, you create an object that can be instantiated into multiple instances with the new keyword, while the literal notation delivers a single object, like a singleton
.
In terms of syntax, object literal notation uses the variable declaration type, while constructor notation uses function declaration type syntax
.
If you want to use private variables or functions in your object, a constructor function is the way to do it. If your object only contains properties and methods, an object literal is fine
.

Can I use visibility modifiers in JavaScript Objects like I can with Java Objects?

No, you cannot use private variables or functions in an object created using object literal notation. All members of an object literal in JavaScript, both properties and functions, are public. The only place where private members can be put is inside of a function
. If you want to use private variables or functions in your object, a constructor function is the way to do it
.

The 6 kinds of functions in JavaScript

There are six kinds of functions in JavaScript:

Regular function: can return anything; always runs to completion after invocation
.
Generator function: returns a Generator object; can be paused and resumed with the yield keyword
.
Arrow function: a shorter syntax for writing function expressions; has a concise syntax and lexical this
.
Function expression: a function that is assigned to a variable; can be anonymous or named
.
Immediately invoked function expression (IIFE): a function that is executed immediately after it is defined; can be used to create a private scope
.
Constructor function: a function that is used to create objects with a specific structure and behavior; uses the new keyword to create instances
.
Understanding the different kinds of functions in JavaScript can help you write more efficient and effective code.

Examples of higher-order functions in JavaScript


Higher-order functions are functions that either accept functions as arguments or return functions as values
. Here are some examples of higher-order functions in JavaScript:
forEach(): a method that calls a function once for each element in an array
.
javascriptlet numbers = [1, 2, 3, 4, 5];numbers.forEach(function(number) { console.log(number);});
map(): a method that creates a new array with the results of calling a function for every element in an array
.
javascriptlet numbers = [1, 2, 3, 4, 5];let doubledNumbers = numbers.map(function(number) { return number * 2;});console.log(doubledNumbers);
filter(): a method that creates a new array with all elements that pass the test implemented by a function
.
javascriptlet numbers = [1, 2, 3, 4, 5];let evenNumbers = numbers.filter(function(number) { return number % 2 === 0;});console.log(evenNumbers);
reduce(): a method that applies a function against an accumulator and each element in the array to reduce it to a single value
.
javascriptlet numbers = [1, 2, 3, 4, 5];let sum = numbers.reduce(function(accumulator, number) { return accumulator + number;}, 0);console.log(sum);
some(): a method that tests whether at least one element in the array passes the test implemented by a function
.
javascriptlet numbers = [1, 2, 3, 4, 5];let hasEvenNumber = numbers.some(function(number) { return number % 2 === 0;});console.log(hasEvenNumber);
every(): a method that tests whether all elements in the array pass the test implemented by a function
.
javascriptlet numbers = [1, 2, 3, 4, 5];let areAllNumbersEven = numbers.every(function(number) { return number % 2 === 0;});console.log(areAllNumbersEven);
By using higher-order functions, you can write more concise and reusable code in JavaScript.

How to create a higher-order function in JavaScript

To create a higher-order function in JavaScript, you can define a function that either accepts a function as an argument or returns a function as its output
. Here is an example of how to create a higher-order function that accepts a function as an argument:
javascriptfunction multiplyByTwo(number) { return number * 2;}
function applyFunctionToArray(array, func) { let result = []; for (let i = 0; i < array.length; i++) { result.push(func(array[i])); } return result;}
let numbers = [1, 2, 3, 4, 5];let doubledNumbers = applyFunctionToArray(numbers, multiplyByTwo);console.log(doubledNumbers);
In this example, multiplyByTwo() is a regular function that multiplies a number by two. applyFunctionToArray() is a higher-order function that accepts an array and a function as arguments. It applies the function to each element in the array and returns a new array with the results. We pass multiplyByTwo() as the function argument to applyFunctionToArray() and get a new array with the doubled numbers.
You can also create a higher-order function that returns a function as its output. Here is an example:
javascriptfunction createMultiplier(multiplier) { return function(number) { return number * multiplier; }}
let double = createMultiplier(2);let triple = createMultiplier(3);
console.log(double(5)); // 10console.log(triple(5)); // 15
In this example, createMultiplier() is a higher-order function that returns a function. It takes a multiplier argument and returns a new function that multiplies a number by the multiplier. We call createMultiplier() with the argument 2 to get a new function that doubles a number. We call it with the argument 3 to get a new function that triples a number. We then call these functions with the argument 5 to get the results.

How to use higher-order functions to simplify code in JavaScript

Higher-order functions can simplify code in JavaScript by allowing you to write more concise and reusable code
. Here are some ways you can use higher-order functions to simplify your code:
Abstraction: Higher-order functions allow you to abstract over actions, not just values. This means you can write functions that take other functions as arguments and perform some action on them. This can help you avoid repeating code and make your code more modular and reusable
.
Built-in higher-order functions: JavaScript provides many built-in higher-order functions like forEach(), map(), filter(), reduce(), some(), and every(). These functions allow you to perform common operations on arrays and other data structures without having to write your own loops or conditionals. By using these functions, you can write more concise and readable code
.
Creating your own higher-order functions: You can also create your own higher-order functions to simplify your code. For example, you can write a function that takes another function as an argument and applies it to each element in an array. This can help you avoid repeating code and make your code more modular and reusable
.
Here is an example of how to use a built-in higher-order function to simplify code:
javascriptlet numbers = [1, 2, 3, 4, 5];let evenNumbers = numbers.filter(function(number) { return number % 2 === 0;});console.log(evenNumbers);
In this example, we use the filter() function to create a new array with only the even numbers from the original array. This is much simpler and more concise than writing a loop and an if statement to achieve the same result.
Here is an example of how to use a custom higher-order function to simplify code:
javascriptfunction applyFunctionToArray(array, func) { let result = []; for (let i = 0; i < array.length; i++) { result.push(func(array[i])); } return result;}
let numbers = [1, 2, 3, 4, 5];let doubledNumbers = applyFunctionToArray(numbers, function(number) { return number * 2;});console.log(doubledNumbers);
In this example, we define a custom higher-order function called applyFunctionToArray() that takes an array and a function as arguments. It applies the function to each element in the array and returns a new array with the results. We use this function to double each number in the original array. This is much simpler and more reusable than writing a loop and a multiplication statement to achieve the same result.

How to use anonymous functions in JavaScript


Anonymous functions in JavaScript are functions that do not have a name associated with them
. They can be used in various ways, such as:
As a function expression: You can define an anonymous function as a function expression and assign it to a variable. This allows you to pass the function as an argument to another function or use it as a callback function. Here is an example:
javascriptlet add = function(a, b) { return a + b;};
let result = add(2, 3);console.log(result); // 5
In this example, we define an anonymous function as a function expression and assign it to the variable add. We then call the function with the arguments 2 and 3 and store the result in the variable result.
As an immediately invoked function expression (IIFE): You can define an anonymous function and immediately invoke it by wrapping it in parentheses and adding a pair of parentheses at the end. This allows you to create a private scope for your code and avoid polluting the global namespace. Here is an example:
javascript(function() { let message = "Hello, world!"; console.log(message);})();
In this example, we define an anonymous function and immediately invoke it. The function defines a variable called message and logs it to the console. Because the function is immediately invoked, the variable is not accessible outside of the function and does not pollute the global namespace.
As a callback function: You can pass an anonymous function as a callback function to another function. This allows you to define the behavior of the callback function inline and avoid defining a separate named function. Here is an example:
javascriptlet numbers = [1, 2, 3, 4, 5];let doubledNumbers = numbers.map(function(number) { return number * 2;});console.log(doubledNumbers);
In this example, we use the map() function to create a new array with each element doubled. We pass an anonymous function as the argument to map() that defines the behavior of the callback function. The function takes a number argument and returns the result of multiplying it by 2.


How to use default parameters in JavaScript functions

Default parameters in JavaScript allow you to initialize named parameters with default values if no values or undefined are passed into the function
. Here are some ways to use default parameters in JavaScript functions:
Basic syntax: You can set a default value for a parameter by assigning it a value in the function declaration. Here is an example:
javascriptfunction greet(name = "World") { console.log(`Hello, ${name}!`);}
greet(); // Hello, World!greet("Alice"); // Hello, Alice!
In this example, we define a function called greet() that takes a name parameter. We set the default value of name to "World". If no argument is passed to the function, the default value is used. If an argument is passed, it overrides the default value.
Using default parameters with other parameters: You can use default parameters with other parameters in the function declaration. Here is an example:
javascriptfunction addNumbers(a, b = 0, c = 0) { return a + b + c;}
console.log(addNumbers(1)); // 1console.log(addNumbers(1, 2)); // 3console.log(addNumbers(1, 2, 3)); // 6
In this example, we define a function called addNumbers() that takes three parameters: a, b, and c. We set the default values of b and c to 0. If no argument is passed for b or c, the default value is used.
Using default parameters with expressions: You can use expressions as default parameter values. Here is an example:
javascriptfunction createPerson(name = "Unknown", age = new Date().getFullYear() - 2000) { return { name: name, age: age };}
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