Javascript- Interview Questions

Interview questions

JavaScript 1) What is JavaScript? JavaScript® (often shortened to JS) is a lightweight, interpreted, interpreted, object-oriented language. 2) What is variable scoping? A variable can be either of global or local scope. A global variable is a variable declared in the main body of the source code, outside all functions, while a local variable is one declared within the body of a function or a block. 3) What is a closure? The function keyword within another function, the inner function has access to variables in the outer function.

Example:  function foo(x) { var tmp = 3; return function (y) { alert(x + y + (tmp)); // will also alert 16  }  } var bar = foo(2); // bar is now a closure. bar(10);

4) What does variable hoisting mean in JavaScript? In JavaScript, you can have multiple var-statements var -statements in a function. All of these statements act as if they were declared at the top of the function. Hoisting is the act of moving the declarations to the top of the function. 5) What is the difference between == and === in JavaScript? J avaScript? == checks equality whereas === checks equality and type. 6) What kind of conditional statements are available in JavaScript? 

IF statement



IF ELSE statement



IF ELSE if statement



SWITCH statement

7) What kind of loops are available in JavaScript? • FOR loop • FOR IN loop • WHILE loop • DO while - This does one full iteration iteration before checking the condition

8) Explain how ‘this’ works in JavaScript? Unlike other object oriented languages "this" does not necessarily relate to the surrounding object, it is the current context of the executing code. Call and apply can be used to change the meaning of "this". 9) Explain what is Prototype in JavaScript? A prototype is an internal object from which other objects inherit properties. Its main purpose is to allow multiple instances of an object to share a common property. 10) How would you change the context of the function By using “call” and “apply” method  - These two simple methods inherent to all

functions allow you to execute any function in any d esired context. The “call 

”  ” 

function requires the arguments to be listed explicitly while the “apply   function  function ”  ” 

allows you to provide the arguments as an array.

11) How do you determine if a JavaScript instance i nstance object was created from a specific constructor? By using instanceof  operator  operator it Checks the current object and returns true if the object is of the specified object type. 12) What are the different JavaScript data types? Give example of each. 

Undefined



Number



String



Boolean



Object



Function



Null

13) What is the difference between a value that is undefined and a value that is null?

Null  is  is actually an assignment value, and is used to attribute an empty value to a variable. A variable is undefined when it’s been declared without an assigned value.

14) Explain how timers work in JavaScript. setTimeout(function, milliseconds): milliseconds): Creates a timer that will call a function after a designated amount of milliseconds. This also generates an id value so the coder can access it another time. setInterval(function, milliseconds): milliseconds): Acts the same way as the setTimer function, only it repeats itself based on the number of milliseconds given. clearInterval(id): clearInterval(id): Used to stop a timer. 15) Is JavaScript case sensitive? Yes, absolutely. For example, the function getElementById getElementById is not the same as the function getElementbyID. getElementbyID. Keeping your capitalization consistent is important. 16) Explain the different types of pop-up boxes you can create in JavaScript. Alert Box: Used Box: Used to confirm that a user understands a vital piece of information before proceeding. The user must click OK to exit the box. Confirm box: Used box: Used when user verification is required. It will return TRUE if the user clicks OK, and FALSE if the user clicks CANCEL. Prompt box: Used box: Used if the user needs to input something before proceeding. When the user inputs a value and presses OK, the prompt box will return the input value. If the user clicks CANCEL without inputting a value, the input value will return as null. 17) What is an Object Literal? An object literal is a comma-separated comma-separated list of name-value pairs wrapped in curly braces. Object literals encapsulate data, data, enclosing it in a tidy package. This minimizes the use of global variables which can cause problems when combining code. Object literal property values can be of any data type, including array literals, functions, and nested object literals. 18) What is a Promise in JavaScript? A promise represents the eventual value returned from the single completion of an operation. A promise may be in one of the three states, unfulfilled, fulfilled, and failed. The promise may only move from unfulfilled to fulfilled or unfulfilled to failed.

19) What is event bubbling and event capturing? Event bubbling and capturing are two ways of event propagation in HTML DOM. In bubbling the event is first captured and handled by the inner most element and then propagated to outer elements. In capturing the event is first captured by the outer most element and propagated to the inner most element. 20) Explain how to handle errors in JavaScript. You can handle runtime exceptions by using try and catch statements. statements. Run the code you want to test in the try block, and if a runtime error occurs, it will execute the code designated in the catch block.