Things to avoid in JavaScript (the bad parts)

Avoid creating a new object by using new Object(). Use the object literal syntax {} instead.
Same thing for arrays, favor [] over new Array().
Avoid blocks except where statements require them (if, switch, loops, try).
Never assign inside an if of while statements condition part
Never use == and !=. Use === and !== instead.
Never use eval. Why? It has performance issues (it runs the interpreter/compiler), it has security issues (code injection if used with user input), difficulties in debugging.
Never use with, as it modifies the scope chain and can be a source of confusion.
Always pass functions to setTimeout and setInterval
Never use Array as an associative arrays, use Object instead. The part of the Array object that provides that functionality is in fact provided by the Object prototype, so you could really have used a Date object for that same thing.
Don’t use \ at the end of a string to create a multiline string, it’s not part of ECMAScript. Use string concatenation ' string1 ' + ' string2 ' instead
Never modify the prototypes of the built-in objects Object and Array. Modify other prototypes of other objects such as Function with caution as it could lead to bugs hard to debug.