Java Script Interview Question and Answer

25. What is purpose of onError event handler in JavaScript?
The onerror event handler was the first feature to facilitate error handling for JavaScript. The error event is fired on the window object whenever an exception occurs on the page.
The onerror event handler provides three pieces of information to identify the exact nature of the error −
Error message − The same message that the browser would display for the given error.
URL − The file in which the error occurred.
Line number − The line number in the given URL that caused the error.

26. What are the variable naming conventions in JavaScript?
While naming your variables in JavaScript keep following rules in mind.
You should not use any of the JavaScript reserved keyword as variable name. These keywords are mentioned in the next section. For example, break or boolean variable names are not valid.
JavaScript variable names should not start with a numeral (0-9). They must begin with a letter or the underscore character. For example, 123test is an invalid variable name but _123test is a valid one.
JavaScript variable names are case sensitive. For example, Name and name are two different variables.

27. What is closure?
Closures are created whenever a variable that is defined outside the current scope is accessed from within some inner scope

28. What is the use of isNaN function?
isNan function returns true if the argument is not a number otherwise it is false.

29. What are global variables? How are these variable declared and what are the problems associated with using 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.

30. What is a prompt box?
A prompt box is a box which allows the user to enter input by providing a text box. Label and box will be provided to enter the text or number.

31. Does JavaScript support automatic type conversion?
Yes JavaScript does support automatic type conversion, it is the common way of type conversion used by JavaScript developers

32. How can you convert the string of any base to integer in JavaScript?
The parseInt() function is used to convert numbers between different bases. parseInt() takes the string to be converted as its first parameter, and the second parameter is the base of the given string.