JavaScript interview questions and answers

  1. What’s relationship between JavaScript and ECMAScript? - ECMAScript is yet another name for JavaScript (other names include LiveScript). The current JavaScript that you see supported in browsers is ECMAScript revision 3.
  2. What are JavaScript types? - Number, String, Boolean, Function, Object, Null, Undefined.
  3. How do you convert numbers between different bases in JavaScript? - Use the parseInt() function, that takes a string as the first parameter, and the base as a second parameter. So to convert hexadecimal 3F to decimal, use parseInt ("3F", 16);
  4. What does isNaN function do? - Return true if the argument is not a number.
  5. What is negative infinity? - It’s a number in JavaScript, derived by dividing negative number by zero.
  6. What boolean operators does JavaScript support? - &&, || and !
  7. What does "1"+2+4 evaluate to? - Since 1 is a string, everything is a string, so the result is 124.
  8. How about 2+5+"8"? - Since 2 and 5 are integers, this is number arithmetic, since 8 is a string, it’s concatenation, so 78 is the result.
  9. What looping structures are there in JavaScript? - for, while, do-while loops, but no foreach.
  10. How do you create a new object in JavaScript? - var obj = new Object(); or var obj = {};
  11. How do you assign object properties? - obj["age"] = 17 or obj.age = 17.
  12. What’s a way to append a value to an array? - arr[arr.length] = value;
  13. What is this keyword? - It refers to the current object.

Check out JavaScript tutorials page for further reading.

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10 Comments on JavaScript interview questions and answers

  1. fester_franz
    Posted 11/10/2006 at 5:23 pm | Permalink

    #9: isn’t there a foreach in the JavaScript 1.6, see:
    http://developer.mozilla.org/en/docs/New_in_JavaScript_1.6#Array_extras

    thanks nonetheless
    – fester_franz

  2. kavitha reddy
    Posted 11/16/2006 at 5:42 am | Permalink

    Plz Urgent,

    If possible give me(in javascript) the code for:

    if a item is selected in drop down list,then dropdown list should dissappear and a textbox should appear…

  3. Janani
    Posted 2/19/2007 at 9:57 pm | Permalink

    question number 13
    This is a keyword used to refer current object, this is used to call constructor of the same class, and super is used to call constructor of the parent class

  4. Backia
    Posted 3/1/2007 at 6:04 am | Permalink

    If possible give me(in javascript) the code for:

    if a item is selected in drop down list,then dropdown list should dissappear and a textbox should appear…

    Hi kavitha reddy, try this…

    New Page 1

    function disappear()
    {
    document.frm1.txt1.value=document.frm1.D1.value;
    document.getElementById("textbox").style.display="block";
    document.getElementById("list").style.display="none";

    }

    one
    two
    three

  5. Spirytus
    Posted 4/2/2007 at 7:33 pm | Permalink

    Regarding question 12.. hasn’t Array object push() method in JavaScript? Its available in ActionScript and if I remember correctly JavaScript had same method push() that appends element to the end of array.

  6. x
    Posted 5/2/2007 at 4:52 am | Permalink

    push() is a Prototype extension to Array object in JavaScript.

  7. Vicky
    Posted 6/30/2007 at 12:51 am | Permalink

    What is JQUERY and how it is more fast and concise than Javascript?

  8. Vicky
    Posted 6/30/2007 at 1:05 am | Permalink

    Hello All:

    If possible then Solve this;

    I have two Drop Downs(select box) and one button.

    1. The first drop down is for Country
    2. Second one is States

    If I select the country US then the states comes in US will appear in states drop down. After that when i select any states from the list then the DIV comes out just below there and again onclick to the button the Div is hide.

    Thsi appear and disappear of DIV is like SHUTTER or Slowly comes down and Slow goes up..

  9. Dillon
    Posted 4/21/2008 at 3:36 am | Permalink

    for the answer:
    What’s a way to append a value to an array?
    Why not -> arr.push(value);

  10. Christopher Parker
    Posted 4/28/2008 at 10:10 am | Permalink

    Array.push was not implemented before IE5.5 and Netscape 4.

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