Pro JavaScript programmer interview questions with answers

Question

What are good questions to determine if applicant is really a pro JavaScript  browser side  developer    Questions that can distinguish if someone is not an ad-hoc JavaScript programmer  but is really doing professional JavaScript development  object-oriented  reusable  and maintainable   Please provide answers  so an intermediate and ad-hoc JavaScript programmers can interview someone more experienced  coming up with answers to quite few of those advanced questions will elude me  Please avoid open questions   Please keep one interview question answer per SO answer for better reading experience and easier interview preparation

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Ask about  this   This is one good question which can be true test of JavaScript developer

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I m assuming you mean browser-side JavaScript   Ask him why  despite his infinite knowledge of JavaScript  it is still a good idea to use existing frameworks such as jQuery  Mootools  Prototype  etc   Answer  Good coders code  great coders reuse  Thousands of man hours have been poured into these libraries to abstract DOM capabilities away from browser specific implementations  There s no reason to go through all of the different browser DOM headaches yourself just to reinvent the fixes

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Ask them how they ensure their pages continue to be usable when the user has JavaScript turned off or JavaScript isn t available   There s no One True Answer  but you re fishing for an answer talking about some strategies for Progressive Enhancement      Progressive Enhancement consists of   the following core principles          basic content should be accessible to all browsers   basic functionality should be accessible to all browsers   sparse  semantic markup contains all content   enhanced layout is provided by externally linked CSS   enhanced behavior is provided by   Unobtrusive   JavaScript unobtrusive    externally   linked JavaScript   end user browser preferences are respected

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Ask how accidental closures might cause memory leaks in IE

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Because JavaScript is such a small language  yet with incredible complexity  you should be able to ask relatively basic questions and find out if they are really that good based on their answers   For instance  my standard first question to gauge the rest of the interview is      In JavaScript  what is the difference between var x   1 and x   1   Answer in as much or as little detail as you feel comfortable    Novice JS programmers might have a basic answer about locals vs globals   Intermediate JS guys should definitely have that answer  and should probably mention function-level scope   Anyone calling themselves an  advanced  JS programmer should be prepared to talk about locals  implied globals  the window object  function-scope  declaration hoisting  and scope chains   Furthermore  I d love to hear about   DontDelete    hoisting precedence  parameters vs var vs function   and undefined   Another good question is to ask them to write a sum   function that accepts any number of arguments  and returns their sum   Then  ask them to use that function  without modification  to sum all the values in an array   They should write a function that looks like this   function sum       var i  l  result   0    for  i   0  l   arguments length  i  lt  l  i          result    arguments i         return result    sum 1 2 3      6   And they should invoke it on your array like this  context for apply can be whatever  I usually use null in that case    var data    1 2 3   sum apply null  data      6   If they ve got those answers  they probably know their JavaScript   You should then proceed to asking them about non-JS specific stuff like testing  workflows  version control  etc  to find out if they re a good programmer

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Ask  What unit testing framework do you use  and why    You can decide if actually using a testing framework is really necessary  but the conversation might tell you a lot about how expert the person is

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Basic JS programmming   Scope of variable What is Associative Array  How do we use it    OOPS JS   Difference between Classic Inheritance and Prototypical Inheritance  What is difference between private variable  public variable and static variable  How we achieve this in JS  How to add remove properties to object in run time  How to achieve inheritance   How to extend built-in objects  Why extending array is bad idea    DOM and JS   Difference between browser detection and feature detection DOM Event Propagation Event Delegation Event bubbling V s Event Capturing   Misc   Graceful Degradation V s Progressive Enhancement

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intermediate programmers should have technical mastery of their tools   if he s passed the technical phone screen-esque questions above  make him sketch out something stupid on the spot  like an ajax url shortner  then grill him on his portfolio  no amazing portfolio   intermediate developer in this domain and not the guy you want in charge of your shiny new project

[javascript] Pro JavaScript programmer interview questions (with answers)

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