JavaScript: The Good Parts Part Eight: Style

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JavaScript The Good PartsPart Eight Style

• Douglas Crockford
• douglas_at_crockford.com

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Bad Parts

• Global Variables
• adds and concatenates
• Semicolon insertion
• typeof
• with and eval
• phony arrays
• and !
• false, null, undefined, NaN

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• value myObjectname
• if (value null)
• alert(name ' not found.')

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• value myObjectname
• if (value undefined)
• alert(name ' not found.')

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Good features that interact badly

• Objects can inherit from other objects.
• Functions can be members of objects.
• for..in statement mixes inherited functions with
  the desired data members.

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for in is troublesome

• Design question Should for..in do a shallow skim
  or a deep dredge?
• Decision Deep dredge. The programmer must
  explicitly filter out the deep members.
• Except They didn't tell anybody!
• Consequence Lots of confusion about how to use
  for..in.

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Bad Heritage

• Blockless statements
• if (foo)
• bar()
• Expression statements
• foo
• Floating point arithmetic
• 0.1 0.2 ! 0.3
• and --
• switch

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Global

• var names 'zero', 'one', 'two',
• 'three', 'four', 'five', 'six',
• 'seven', 'eight', 'nine'
• var digit_name function (n)
• return namesn
• alert(digit_name(3)) // 'three'

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Slow

• var digit_name function (n)
• var names 'zero', 'one', 'two',
• 'three', 'four', 'five', 'six',
• 'seven', 'eight', 'nine'
• return namesn
• alert(digit_name(3)) // 'three'

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Closure

• var digit_name (function ()
• var names 'zero', 'one', 'two',
• 'three', 'four', 'five', 'six',
• 'seven', 'eight', 'nine'
• return function (n)
• return namesn
• ())
• alert(digit_name(3)) // 'three'

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Style Isn't Subjective

• block
• ....
• Works well in JavaScript

• block
• ....
• Might work well in other languages

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Style Isn't Subjective

• return
• ok true
• Works well in JavaScript

• return
• ok false
• SILENT ERROR!

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Style Isn't Subjective

• return
• ok false

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Style Isn't Subjective

• return // semicolon insertion
• ok false

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Style Isn't Subjective

• return
• // block
• ok false

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Style Isn't Subjective

• return
• ok false // label

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Style Isn't Subjective

• return
• // useless
• ok false // expression
• // statement

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Style Isn't Subjective

• return
• ok false // semicolon
• // insertion

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Style Isn't Subjective

• return
• ok false
• // empty statement

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Style Isn't Subjective

• return
• // unreachable statement
• ok false

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Style Isn't Subjective

• return
• ok false
• Bad style

• return
• ok false
• Bad results

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It is possible to write excellent programs in
JavaScript.

• It requires discipline.

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Unlearning Is Really Hard

• Perfectly Fine Faulty

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It's not ignorance does so much damage it's
knowin' so derned much that ain't so.

• Josh Billings

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Write Code to be Read

• Conventions of spacing, punctuation, and
  formatting are as important in good programming
  as in good writing.

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Recommended Reading

• Code Conventions for the JavaScript Programming
  Language
• http//javascript.crockford.com/code.html
• The Elements of JavaScript Style
• http//javascript.crockford.com/style1.html
• http//javascript.crockford.com/style2.html