Linting fixes and simplification

These changes fix issues detected by JSHint and simplify expressions to
increase readability.

Fixed linting issues (96 in total, jshint@2.7.0):
* Accidentally globally defined variables
* Redefined variables
* Typo in identifier name
* Duplicate object key
* Coercing equal
* Confusing negation of 'lesser than' comparison
* Undefined globals from libraries
* Missing blocks for conditions
* Functions defined inside of loops (performance)

Simplified expressions:
* Check for thruthy/falsy values when possible instead of type checking
  against 'undefined'
* Use logical OR for fallback values (instead of `x = x ? x : y;`)
* Remove explicit type conversion in conditions (`if (!!x)`)
  and in places where the variable already is a boolean
* Replace ternary IF with a boolean result by the condition itself
* Refactored:
  ```
  if (newValue === 0 &&
      typeof this.someProperty === "undefined" ||
      newValue === ...