[javascript/en] Spacing and capitalization of comments

... and a few grammar fixes.

javascript.html.markdown

@@ -136,7 +136,7 @@ undefined; // used to indicate a value is not currently present (although
 // character.
 var someVar = 5;
 
-// if you leave the var keyword off, you won't get an error...
+// If you leave the var keyword off, you won't get an error...
 someOtherVar = 10;
 
 // ...but your variable will be created in the global scope, not in the scope
@@ -145,7 +145,7 @@ someOtherVar = 10;
 // Variables declared without being assigned to are set to undefined.
 var someThirdVar; // = undefined
 
-// if you want to declare a couple of variables, then you could use a comma 
+// If you want to declare a couple of variables, then you could use a comma 
 // separator
 var someFourthVar = 2, someFifthVar = 4;
 
@@ -223,15 +223,15 @@ for (var i = 0; i < 5; i++){
     // will run 5 times
 }
 
-//The For/In statement loops iterates over every property across the entire prototype chain
+// The for/in statement iterates over every property across the entire prototype chain.
 var description = "";
 var person = {fname:"Paul", lname:"Ken", age:18};
 for (var x in person){
     description += person[x] + " ";
 }
 
-//If only want to consider properties attached to the object itself,
-//and not its prototypes use hasOwnProperty() check
+// To only consider properties attached to the object itself
+// and not its prototypes, use the `hasOwnProperty()` check.
 var description = "";
 var person = {fname:"Paul", lname:"Ken", age:18};
 for (var x in person){
@@ -240,8 +240,9 @@ for (var x in person){
     }
 }
 
-//for/in should not be used to iterate over an Array where the index order is important.
-//There is no guarantee that for/in will return the indexes in any particular order
+// For/in should not be used to iterate over an Array where the index order
+// is important, as there is no guarantee that for/in will return the indexes
+// in any particular order.
 
 // && is logical and, || is logical or
 if (house.size == "big" && house.colour == "blue"){
@@ -256,7 +257,7 @@ var name = otherName || "default";
 
 
 // The `switch` statement checks for equality with `===`.
-// use 'break' after each case
+// Use 'break' after each case
 // or the cases after the correct one will be executed too.
 grade = 'B';
 switch (grade) {