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| -rw-r--r-- | javascript.html.markdown | 28 |
1 files changed, 21 insertions, 7 deletions
diff --git a/javascript.html.markdown b/javascript.html.markdown index cc279b9a..865edc36 100644 --- a/javascript.html.markdown +++ b/javascript.html.markdown @@ -104,9 +104,10 @@ false // There's also null and undefined null // used to indicate a deliberate non-value -undefined // used to indicate a value that hasn't been set yet +undefined // used to indicate a value is not currently present (although undefined + // is actually a value itself) -// null, undefined, NaN, 0 and "" are falsy, and everything else is truthy. +// false, null, undefined, NaN, 0 and "" are falsy, and everything else is truthy. // Note that 0 is falsy and "0" is truthy, even though 0 == "0". /////////////////////////////////// @@ -142,7 +143,7 @@ myArray[1] // = 45 // JavaScript's objects are equivalent to 'dictionaries' or 'maps' in other // languages: an unordered collection of key-value pairs. -{key1: "Hello", key2: "World"} +var myObj = {key1: "Hello", key2: "World"} // Keys are strings, but quotes aren't required if they're a valid // JavaScript identifier. Values can be any type. @@ -211,9 +212,13 @@ function myFunction(thing){ myFunction("foo") // = "FOO" // Functions can also be defined "anonymously" - without a name: -function(thing){ +(function(thing){ return thing.toLowerCase() -} +}) +// Note: Stand-alone function declarations require an identifier name. +// Anonymous functions are values, not declarations, so they must be +// treated as a value. We wrap ours here in ( ) to do so, or you could assign +// it to a variable, or pass it as a parameter to another function. // (we can't call our function, since we don't have a name to refer to it with) // JavaScript functions are first class objects, so they can be reassigned to @@ -247,12 +252,15 @@ i // = 5 - not undefined as you'd expect in a block-scoped language // in a web browser is always 'window'. The global object may have a // different name in non-browser environments such as Node.js. window.permanent = 10 - // Or, as previously mentioned, we can just leave the var keyword off. + // Or, as previously mentioned, if you leave the var keyword off, a + // global variable will be created when you assign it a value. However, + // this behavior is frowned upon, and in fact is disallowed in "strict + // mode". permanent2 = 15 })() temporary // raises ReferenceError permanent // = 10 -permanent2 // = 15 +permanent2 // = 15 <-- the accidental global variable // One of JavaScript's most powerful features is closures. If a function is // defined inside another function, the inner function has access to all the @@ -268,6 +276,12 @@ function sayHelloInFiveSeconds(name){ // access to the value of prompt. } sayHelloInFiveSeconds("Adam") // will open a popup with "Hello, Adam!" in 5s +// inner() has access to the variable "prompt" strictly because of lexical scope. +// A closure is being demonstrated because the inner() function is being executed +// at a later time, and in fact being executed "outside" the scope where it was +// declared (inside of the implementation of setTimeout()), but yet inner() STILL +// has access to the variable "prompt". It is said that inner() has a "closure" +// over the variables of sayHelloInFiveSeconds(). /////////////////////////////////// // 5. More about Objects; Constructors and Prototypes |