Top Six Things Your Grandmother Doesn't Know About JavaScript

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Top Six Things Your Grandmother Doesn't Know About JavaScript

1. It will sort things lexicographically and not tell you

   [5, 1, 10].sort();
   // => [ 1, 10, 5 ]
   // Numbers go in, numbers come out. You can't explain it!
   
   [5, 1, 10].sort(function (a, b) { return a - b; });
   // => [ 1, 5, 10 ]
   // Requires a custom comparator to work properly

2. isNaN does not actually tell you if things are numbers; it only distinguishes between NaN and !NaN

   isNaN(NaN);
   // => true
   isNaN(2);
   // => false
   isNaN(true);
   // => false
   isNaN(null);
   // => false

   Oh, wait, no, that's not how it works at all

   isNaN(undefined);
   // => true
   isNaN({});
   // => true

   Maybe use Number.isNaN()?

3. Addition is non-commutative

   {} + [];
   // => 0
   [] + {};
   // => '[object Object]'

4. Hoisting: not just for pirate flags!

   // You would think this would be a ReferenceError. You would be wrong!
   
   console.log(a);
   
   var a = 'Hello, world!';
   
   // Prints `undefined`; JS hoists `var a;` to the top of the current scope,
   // but the assignment is still done below

5. Everything is an object

   3.toString();
   // => SyntaxError: Unexpected token ILLEGAL

   ...if you know how to ask

   3..toString();
   // => '3'

6. IEEE 754 floating point numbers are neat

   999999999999999999999999 === 1000000000000000000000000;
   // => true

I should include implied globals:

foo = 'string'; // Global!

And calling constructors without new is just... bad times

this one really made me angry when i found out.

  [].push(5) // 1 (the new length of the array! cause JavaScript)