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Kyle Simpson @getify http://getify.me • • •
LABjs grips asynquence
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Kyle Simpson @getify http://getify.me
http://speakerdeck.com/getify
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http://YouDontKnowJS.com
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https://developer.mozilla.org/en-US/docs/JavaScript https://github.com/rwldrn/idiomatic.js http://www.ecma-international.org/ecma-262/5.1/ https://github.com/tc39/ecma262#current-proposals
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Agenda Values & Types • • •
Primitive Types Natives Coercion
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• • • • • • •
undefined string number boolean object function null Primitive Types
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Primitive Types (literals)
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Primitive Types (literals)
Quiz
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Primitive Types
Quiz
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Primitive Types
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• • • • •
NaN (“not a number”) Infinity, -Infinity null undefined (void) +0, -0 Primitive Types: special values
NaN
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Primitive Types: special values
NaN
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Primitive Types: special values
NaN
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Primitive Types: special values
Negative Zero
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Primitive Types: special values
Negative Zero
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Primitive Types: special values
ES6: Object.is(..)
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Primitive Types: special values
Quiz
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Primitive Types: special values
Quiz
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Primitive Types: special values
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• • • • • • • • •
String Number Boolean Function Object Array RegExp Date Error
Native Types? Native Objects?
Natives
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• • • • • • • • •
String() Number() Boolean() Function() Object() Array() RegExp() Date() Error()
Native Functions?
Natives
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• • • • • • • • •
new new new new new new new new new
String() Number() Boolean() Function() Object() Array() RegExp() Date() Error()
Native Constructors
Natives
Quiz
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Natives
Quiz
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Natives
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Natives: literals
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Natives: literals
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Coercion
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Abstract Operations
Coercion
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ToString
Coercion: Abstract Operations
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null undefined true false 3.14159 0 -0
"null" "undefined" "true" "false" "3.14159" "0" "0" Coercion: ToString
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ToString: toString()
Coercion: ToString
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[] [1,2,3] [null,undefined] [[[],[],[]],[]] [,,,,]
"" "1,2,3" "," ",,," ",,,"
Coercion: ToString (Array)
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{} "[object Object]" {a:2} "[object Object]"
Coercion: ToString (Object)
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ToNumber
Coercion: Abstract Operations
"" "0" "-0" " 009 " "3.14159" "0." ".0" "." "0xaf"
0 0 -0 9 3.14159 0 0 NaN 175
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Coercion: ToNumber
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false true null undefined
0 1 0 NaN
Coercion: ToNumber
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ToNumber (object): ToPrimitive (number)
Coercion: ToNumber (Array/Object)
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ToPrimitive
Coercion: Abstract Operations
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valueOf() toString()
Coercion: ToPrimitive
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[""] ["0"] ["-0"] [null] [undefined] [1,2,3] [[[[]]]]
0 0 -0 0 0 NaN 0
Coercion: ToNumber/ToPrimitive (Array)
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ToBoolean
Coercion: Abstract Operations
Falsy
Truthy
“” 0, +0, -0 null NaN false undefined
“foo” 23 { a:1 } [1,3] true function(){..} ...
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Coercion: ToBoolean
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Explicit
Implicit
Coercion
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Explicit: it’s obvious from the code that you’re doing it.
Explicit Coercion
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string
number
Explicit Coercion
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Explicit Coercion
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*
boolean
Explicit Coercion
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Explicit Coercion
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Explicit (or Implicit?) Coercion
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Implicit: happens as a side effect of some other operation
Implicit Coercion
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Implicit: evil?
Implicit Coercion
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Implicit Coercion
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Implicit Coercion
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Implicit Coercion
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7/24 17/24
Implicit Coercion
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NEVER use == false or == true!
Implicit Coercion: The Bad Parts
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Ask Yourself: 1. Can either value be true or false? 2. Can either value ever be [], “”, or 0?
Implicit vs Explicit Coercion
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Implicit Coercion: The Safe Parts
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Primitive
Native
Implicit Coercion
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Implicit Coercion
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"Any sufficiently advanced technology is indistinguishable from magic." --Arthur C. Clarke
Implicit Coercion
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"Any sufficiently complex technology is indistinguishable from magic." --many devs
Implicit Coercion
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"Any sufficiently confusing technology is indistinguishable from magic." --many devs
Implicit Coercion
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== vs. === Implicit Coercion
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== checks value === checks value and type
== allows coercion === disallows coercion Implicit Coercion
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Implicit Coercion: == vs. ===
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Implicit Coercion
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http://davidwalsh.name/fixing-coercion Number("") === 0 Number(false) === 0 Number(true) === 1 Number(null) === 0 String([]) === "" String([null]) === "" String([undefined]) === "" Value Coercion: The Bad Parts
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http://getify.github.io/coercions-grid/
Value Coercion: The Bad Parts
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but... but... Implicit Coercion: == helpful?
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Implicit Coercion: == helpful?
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Implicit Coercion: == helpful?
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What about the performance of == or ===?
Implicit Coercion: == vs. ===
http://jsperf.com/triple-equals-vs-double-equals/5
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Implicit Coercion: == vs. === performance
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Implicit Coercion: == vs. === performance
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#FUD: using == in your code is dangerous. Avoid it!
Implicit Coercion: == vs. ===
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#protip: use === where it’s safer and use == where it’s more helpful.
Implicit Coercion: == vs. ===
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#FUD: JavaScript’s implicit coercion is a flaw in the design of the language. Avoid it!
Coercion
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#protip: use explicit coercion where it’s safer and use implicit coercion where it’s more helpful.
Coercion
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http://jscoercion.qfox.nl/
Coercion
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Kyle Simpson @getify http://getify.me Thanks! Questions?
