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JavaScript programs should be stored in and delivered as .js files
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script tags should be placed as late in the body as possible, There is no need to use the language or type attributes
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The unit of indentation is four spaces
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Avoid long lines
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Be generous with comments
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It is preferred that each variable be given its own line and comment, They should be listed in alphabetical order if possible, variables that are only declared but do not get a value assigned may be declared on the same line
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When a function is to be invoked immediately, the entire invocation expression should be wrapped in parens
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The with statement should not be used
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Use the === and !== operators
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eval is Evil
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Do not pass strings to setTimeout or setInterval
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Function Declarations Within Blocks: No
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Always preferred over non-standards features(e.g. string.charAt(3) over string[3])
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In case a variable contains a jQuery object, the variable can start with $
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Variables should not be defined in the global scope
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Use the module pattern to encapsulate
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Namespace your JavaScript if you need to refer to it elsewhere
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When optimizating, focus on the big things
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Excessive DOM changes that force the page to re-render
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Events that get fired all the time (for example, resizing/scrolling)
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Lots of HTTP requests (and even this is becoming less important)
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Lazy load assets that aren't immediately required
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Causing excessive document reflows(DOM modification is slow)
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Going overboard with file concatenation
see more: JavaScript Style Guide, Best Practices，Code Conventions
