Warn

• About debugging code
• About unsafe for..in
• About == null
• About arguments.caller and .callee
• About empty blocks
• About unsafe comparisons
• About assignments inside if/for/...
• About functions inside loops

• About eval
• About unsafe line breaks
• When bitwise operators are used
• When code is not in strict mode
• When variable is undefined
• When blocks omit {}
• When new is used for side effects

Assume

• Browser
• Development (console, etc.)
• jQuery
• EcmaScript 5
• NodeJS