Most frequently asked JavaScript interview questions with their answers that every beginner to advance JavaScript Developer should know are as follow.
Beginner Level Javascript Interview Questions
Here is part one of the javascript interview questions. bookmark this page and stay tuned for more beginner to advance interview questions to gauge your javascript level.
How many ways can you declare an array in JavaScript?
There are basically two ways to declare an array.
Array literal
new keyword
What is the use of a window in JavaScript?
The window object is a global JavaScript object. The window object is automatically created by the browser that represents a window of a browser. It is used to display the popup dialog box such as an alert dialog box, confirm dialog box, input dialog box, etc.
What are the JavaScript data types?
There are two types of data types in JavaScript:
Primitive Data Types e.g. String, Number, and Boolean are primitive data types.
Non-primitive Data Types Object, Array, and Function (which are all types of objects) are Non-primitive data types
What is the difference between undefined value and null value?
Undefined means a variable has been declared but has not yet been assigned a value. Undefined is a type itself (undefined).
Null value, on the other hand, is a value that is explicitly specified by the keyword “null”. A null is an object.
How you can throw a user-defined exception in JavaScript?
The 'throw'statement throws a user-defined exception in JavaScript.
For example
What are the advantages of using ‘use strict’ at the beginning of a JavaScript source file?
Some of the key benefits of using “use strict” at the top of the JS source file are:
- Easy Debugging, Code flaws that might otherwise have been ignored or would have failed silently will generate an error or throw exceptions.
- Prevents accidental globals, Without strict mode, assigning a value to an undeclared variable automatically creates a global variable with that name. This is one of the most common errors in JavaScript. In strict mode, attempting to do so throws an error.
- Eliminates this coercion, Without strict mode, a reference to this value of null or undefined is automatically coerced to the global. In strict mode, referencing this value of null or undefined throws an error.
- Disallows duplicate parameter values, Strict mode throws an error when it detects a duplicate named argument for a function (e.g., function foo(val1, val2, val1){}), thereby catching what is almost certainly a bug in your code that you might otherwise have wasted lots of time tracking down.
What is the difference between JavaScript and Java?
Java is a programming language. However, JavaScript is a scripting language that can be used with HTML as well. Both Java and JavaScript are independent languages and are designed for different purposes. Java is an object-oriented programming (OOPS) or structured programming language, like C++ or C whereas JavaScript is a scripting language.
What is isNaN in Javascript?
isNan function is used to check if the given value is a number or not.
What is the negative infinity in JavaScript?
Divide by Zero error is called negative infinity in Javascript
What are JavaScript global variables? How are these variables declared and what problems are associated with them?
Global variables are those that are available throughout the length of the code, that is, these have no scope. The var keyword is used to declare a local variable or object. If the var keyword is omitted, a global variable is declared.
Example:
// Declare a global this_is_global_variable= “Test”;
The problems that are faced by using global variables are the clash of variable names of local and global scope. Also, it is difficult to debug and test the code that relies on global variables.
What is a Javascript prompt?
A JS prompt is a prompt box that allows the user to enter input by providing a text box. Label and box will be provided to enter the text or number.
What is the strict equality operator “===”?
“ === ” is called a strict equality operator which returns true when the two operands are having the same value and same data type.
What is the function of the delete keyword in JavaScript?
The delete keyword is used to delete the property as well as its value.
What are all the types of Pop up boxes available in JavaScript?
Alert
Confirm and
Prompt
Learn about javascript throw here.
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