Privacy Policy and Terms of Use. In ES6, you can even use functions as the default parameter values. We are going to look over an example where we get a non-satisfactory outcome due to missing arguments and then we will fix it using traditional and default parameters. In ES6, a new feature was introduced which was the default parameters. How to set default values when destructuring an object in JavaScript ? The syntax is as: For the first function call, it used the default value doe for the parameter lastName and for the second it used both the arguments. JavaScript | Passing parameters to a callback function, JavaScript | Know the value of GET parameters from URL. Backbone.js Model.PreviousAttributes() Method. If we were to solve the same problem using the default parameters, the solution would be much more concise like: As you can see, instead of writing numerous lines, we did it in a single line and the output is exactly the same with no errors and no NaN. A Javascript Developer & Linux enthusiast with 4 years of industrial experience and proven know-how to combine creative and usability viewpoints resulting in world-class web applications. Before ECMAScript 2015 (ES6), a parameter's default value could be assigned in the following way: ES6 provided a new syntax where the condition and reassignment depicted above is no longer necessary: This also shows that if a parameter is missing when the function is invoked, its value is kept as undefined, as it can be confirmed by explicitly providing it in the following example (using an arrow function): The default parameters' values are not restricted to numbers, strings or simple objects. The issue of missing arguments in the invoking function call is one of such corners and the default parameter is the guard that we use to save us from unwanted outcomes. Inline operations are allowed when assigning a default value to a parameter. However, it may be useful to set a different default value. The trivial approach can be used with the following lines of code for a function to add 2 numbers: Even though we are not passing any arguments when calling the function inside the console.log() function, we wont get any error. Get access to ad-free content, doubt assistance and more! How to pass two parameters to EventEmitter in Angular 9 ? Using the same function that we created in the above example, for the first case when both the arguments are present then the values inside the function call are used like so: And for the second case, in which one of the arguments was missing, the function used the value that we defined as the default value within the parameter like: Typically when no argument is passed then the parameter is considered to be undefined by JavaScript. Please use ide.geeksforgeeks.org, The above will meet an error since its trying to access the parameter b even before it is initialized. But first, lets learn a little more about the default parameters. How to set align-self property to its default value in CSS ? How to align item set to its default value in CSS ? Example 2: If we do not pass a number as the second parameter and take the default parameter as the second parameter, it will multiply the first number with the default number, and if we pass two numbers as parameters, it will multiply the first number with the second number. Default parameters are the parameters that have defined their default values in case they are missing from the invoking function call. Difference between var and let in JavaScript, Hide or show elements in HTML using display property. How to declare the optional function parameters in JavaScript ? The function getSome() will take 2 parameters, if there is only one argument then it will assign the second parameter twice the value of the first argument. acknowledge that you have read and understood our, GATE CS Original Papers and Official Keys, ISRO CS Original Papers and Official Keys, ISRO CS Syllabus for Scientist/Engineer Exam. Complete Interview Preparation- Self Paced Course. I have experience working with Vue, React & Node.js & currently working on article writing and video creation. A-143, 9th Floor, Sovereign Corporate Tower, We use cookies to ensure you have the best browsing experience on our website. Resulting in the use of earlier defined parameters in the default value of later parameters. How to force Input field to enter numbers only using JavaScript ? Example 3: Default Parameter with Constructor: we can use the default parameter concept with the constructor of a class. How to add HTML elements dynamically using JavaScript ? How to trigger a file download when clicking an HTML button or JavaScript? 
How to set the default value for an HTML
As you can see we can handle the issue of missing arguments like this as well.
How to get URL Parameters using JavaScript ? Convert a string to an integer in JavaScript, Check if an array is empty or not in JavaScript. How do you run JavaScript script through the Terminal? By using our site, you Variables existing in the same scope of the function being declared can be used in its default values. How to set default value to align content in CSS ? 
Convert URL parameters to a JavaScript Object. How to set textarea scroll bar to bottom as a default using JavaScript/jQuery ? How to Create Query Parameters in JavaScript ? JavaScript function parameters are defined as undefined by default. Writing code in comment? How to get specific key value from array in PHP . Come write articles for us and get featured, Learn and code with the best industry experts. 1309 S Mary Ave Suite 210, Sunnyvale, CA 94087
Since, the parameters are initialized from left to right, if we use a later parameter as the default value of an earlier parameter then it will result in an error. To avoid meeting an error because of missing arguments JavaScript added this feature. those that are explicitly provided when the function is invoked: Get monthly updates about new articles, cheatsheets, and tricks. How to Use the JavaScript Fetch API to Get Data? How to get value of selected radio button using JavaScript? How to compare two arrays in JavaScript ? Try out the following lines of code: As you can see, we are giving the function getValue(num) as a default parameter for b and we are passing the parameter a inside it. How to set default values for Angular 2 component properties? How to remove a character from string in JavaScript ? How to Open URL in New Tab using JavaScript ? How to append HTML code to a div using JavaScript ? How to convert Set to Array in JavaScript? To avoid this undesirable outcome, we use default Parameters. How to calculate the number of days between two dates in javascript? A previously declared parameter can be reused as a default value for the upcoming parameters' values. How to set default value in select using ReactJS ? The traditional or trivial approach to avoid an unwanted output is to check the parameter for an undefined value. That is where default parameters come into play. How set default timezone in Node.js for Windows ? Linux Hint LLC, [emailprotected]
This modified text is an extract of the original, Bitwise Operators - Real World Examples (snippets), Function Arguments, "arguments" object, rest and spread parameters, Functions with an Unknown Number of Arguments (variadic functions), How to make iterator usable inside async callback function, Same Origin Policy & Cross-Origin Communication, Using javascript to get/set CSS custom variables. Function parameters with default values are initialized with default values if they contain no value or are undefined. When defining the parameter of the function simply put the default value after the = sign. How to serialize an object into a list of URL query parameters using JavaScript ?
When developing a full-stack application, we usually want to cover all of our ends that can return us any error. How to create an image element dynamically using JavaScript ? How to get character array from string in JavaScript? How to add an object to an array in JavaScript ? How to pass query parameters with a routerLink ? This is because the parameters are initialized from left to right. How to set a default parameter value for a JavaScript function ? Set the value of an input field in JavaScript. The getValue(num) multiples the argument passed to it by 2 and returns the value. In ES6, you can set a default parameter using the parameter before it. Example 1: If we multiply two numbers in this example without passing a second parameter and without using the default parameter, the answer that this function will return is NAN(Not a Number), since if we do not pass the second parameter, the function will multiply the first number with undefined. Parameters define the requirements for invoking a function. Functions can be invoked in order to provide their return value into a default value. What is spread, default and rest parameters in JavaScript ? How to read a local text file using JavaScript? Difference between TypeScript and JavaScript, Form validation using HTML and JavaScript, Differences between Functional Components and Class Components in React. generate link and share the link here. Oftentimes, we invoke a function, but we miss incorporating some arguments that are necessary for the output, and the JavaScript function either gives us an error or returns a NaN value. A function can also be set as the default value callback = function(){}: There are certain characteristics of the operations that can be performed through default values: The arguments array object only retains the parameters whose values are not default, i.e. Declaring default parameters is rather easy. Default parameters are used to put a default value of an argument at the time of writing that function. That is why, when we pass 2 inside calc() function as a single parameter we get the following out: Thats it for default parameters in JavaScript. How to get parameters from a URL string in PHP?