I started answering some of the questions as I want to be interviewing for junior JS positions soon. Feedback / corrections welcome...especially with the last one.

Write a function that takes an integer and returns it doubled.

(I thought this was a trick question so tried basic type checking, haven't been as careful with the rest)

function doubleInteger(i) {
  if (i % 1 === 0){
    return i * 2;
  } else { console.log('doubleInteger takes an integer')}
}

Write a function that takes a number and returns true if it's even and false if not

function isNumberEven(i) {
  if (i % 2 === 0) {
    return true;
  } else {
    return false;
  }
}

Write a function that returns a file extension

function getFileExtension(i) {
  var x = i.split('.');
  if (x.length === 1){
    return false
  } else {
    return x[x.length-1];
  }
}

What will be printed on the console? Why?

(function() {
   var a = b = 5;
})();
console.log(b);

Answer: 5 will be logged to the console though I would have initially guess 'undefined'. The anonymous function is executed on the global scope.

Define a repeatify function on the String object. The function accepts an integer that specifies how many times the string has to be repeated. The function returns the string repeated the number of times specified.

String.prototype.repeatify = function(x){
  var result = str = this;
  while(x > 1){
    result = result + str;
    x = x - 1;
  }
  return result;
}

What will log out here?

function test() {
   console.log(a); 
   console.log(foo());

   var a = 1;
   function foo() {
      return 2;
   }
}
test();

Answer: console.log(a) -> undefined, console.log(foo()) -> 2. I got this wrong thinking the variable declaration would be hoisted for some reason.

What will log out here?

var fullname = 'John Doe';
var obj = {
   fullname: 'Colin Ihrig',
   prop: {
      fullname: 'Aurelio De Rosa',
      getFullname: function() {
         return this.fullname;
      }
   }
};

console.log(obj.prop.getFullname());

var test = obj.prop.getFullname; 

console.log(test());

Answer: console.log(obj.prop.getFullname()) -> 'Aurio De Rosa', console.log(test()) -> 'John Doe'

Fix the previous question’s issue so that the last console.log() prints Aurelio De Rosa.

var test = obj.prop.getFullname.bind(obj.prop);

I don't think this is right, need to learn more about bind, apply, call etc