JavaScript Tutorials

Pre-defined functions

There are a number of functions that are built into the JavaScript engine and available for us to use. Let’s take a look at some of them. While doing so, you’ll have a chance to experiment with functions, their parameters and return values , and become comfortable in working with them. 

For example:


The alert() function helps us to display a pop-up dialog box with an optional message text. It is important to note that it waits for the user to close or dismiss the pop-up dialog box.

alert("Welcome to Edupoly");
//alerts "Welcome to Edupoly"

var x = 20;
var y = 30;
alert(x + y);
//alerts 50


The console.log() function helps us to print some message or value in the console.

//prints "Edupoly" in the console

//prints 3 in the console

var x = 12;
var y = 10;
console.log(x + y);
//prints 22 in the console

console.log("Edu" + "poly");
//prints "Edupoly" in the console

console.log("Good" + " " + "Day");
//prints "Good Day" in the console


The confirm() function displays a confirm dialog box and waits for the user to respond to it. Users can either confirm or cancel/dismiss the dialog box. It returns true or false according to the user response.

confirm("Are you sure you want to logout?");
//opens a confirm dialog box
//returns true if user clicks on confirm
//returns false if user clicks on cancel


The prompt() function opens a dialog box with a message(if we want) and allows the user to input some value. It waits for the user to provide some value and returns the value as a string. It also has a ‘cancel’ option to dismiss the dialog box.

prompt("Enter a number");


parseInt() takes any type of input (most often a string) and tries to make an integer out of it. If it fails, it returns NaN.



parseFloat() is the same as parseInt() but it also looks for decimals when trying to figure out a number from your input.



1. Write a program to alert “Hello! Good morning”.

2. Write a program to print your full name in the console.

3. Write a program to print the product of 10 and 20 in the console.

4. Write a program to open a confirm box that asks “Do you want to delete this item?”.

5. Write a program to open a confirm box that asks “Do you want to delete this item?”. If the user confirms it, it should alert “Item deleted”.

6. Use a prompt box that asks the user to enter a number and print the number in the console.

7. Use a prompt box to take a decimal number input from the user and print only the integer part of the number using parseInt().


Click on this link for MCQs on JavaScript pre-defined functions: Pre-defined functions MCQs