JavaScript Is Neat

Upon further inspection (after spending many hours writing JavaScript) I've discovered that the language is neat, mostly because of the way it is.

Adding Booleans Together

Not sure who would ever do this but nonetheless it is valid.

const value = true + true - false;
console.log(value);

Answer: 2

When adding two booleans together true gets converted to 1, and false gets converted to 0 so: 1 + 1 - 0 = 2

The Infamous Semicolon

const foobar = function() {
  return
  {
    name: 'jimmy'
  }
}
foobar();

I'm sure everyone has encountered this one but it's something that may get overlooked if you're getting quizzed on your skills and you're crunched for time.

This obviously returns undefined because the line with the return statement gets a semicolon added to it and the function it basically returns nothing.

Are Strings Strings?

"hey I like to code" instanceof String

This one got me pretty good. Seems logical that a string is a string... but without explicitly calling the new String() constructor it is actually just an object and the above code returns false.

new String('js is fun') instanceof String

But this will obviously return true.

Clearing an Array

const arr = ['javascript', 'html', 'css'];
arr.length = 0;
console.log(arr[2]);

What do you expect to log to the console? A bit of an odd one but you can actually empty the contents of an array by setting it's length to 0. So trying to index into the array will produce undefined as the array would now be empty.

What is NaN?

NaN in JavaScript sure is neat. It stands for 'not a number' and indicates the value is not a legal number. So since NaN is not-a-number than what is it? It's a number.

typeof NaN 

Returns "number". Nice.

NaN === NaN

Returns false. Great.

So use the isNan() function to check for NaN.

Hoisting

What do you expect to be returned by this function?

function hoisting() {
  return name;
  name = 'taylor';
  function name() { }
  name = 0;
}

If you're like me your initial instinct was to think that the function returned undefined because it just returns right away and jumps out of the code block, but that is incorrect.

The function returns function due to a phenomenon called hosting. All defined variables in an execution context are 'hoisted' to the top including function declarations.

function hoisting() {
  return name;
  name = 'taylor';
  const name = function() { }
  name = 0;
}

This code, on the other hand, returns undefined because of the function expression is being hoisted and has no value due to the function returning before the value is set.