diff options
| -rw-r--r-- | javascript.html.markdown | 36 |
1 files changed, 18 insertions, 18 deletions
diff --git a/javascript.html.markdown b/javascript.html.markdown index cc279b9a..9b87b022 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. @@ -184,7 +185,7 @@ do { input = getInput() } while (!isValid(input)) -// the for loop is the same as C and Java: +// the for loop is the same as C and Java: // initialisation; continue condition; iteration. for (var i = 0; i < 5; i++){ // will run 5 times @@ -210,12 +211,6 @@ function myFunction(thing){ } myFunction("foo") // = "FOO" -// Functions can also be defined "anonymously" - without a name: -function(thing){ - return thing.toLowerCase() -} -// (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 // different variable names and passed to other functions as arguments - for // example, when supplying an event handler: @@ -224,9 +219,16 @@ function myFunction(){ } setTimeout(myFunction, 5000) +// Functions can also be defined "anonymously" - without a name: +var lowerFunction = function(thing){ + return thing.toLowerCase() +} +lowerFunction("Foo") // = "foo" +// (note: we've assigned our anonymous function to a variable - if we didn't, we +// wouldn't be able to access it) + // You can even write the function statement directly in the call to the other // function. - setTimeout(function myFunction(){ // this code will be called in 5 seconds' time }, 5000) @@ -247,25 +249,23 @@ 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. - permanent2 = 15 })() temporary // raises ReferenceError permanent // = 10 -permanent2 // = 15 // 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 -// outer function's variables. +// outer function's variables, even after the outer function exits. function sayHelloInFiveSeconds(name){ var prompt = "Hello, " + name + "!" function inner(){ alert(prompt) } setTimeout(inner, 5000) - // setTimeout is asynchronous, so this function will finish without waiting - // 5 seconds. However, once the 5 seconds is up, inner will still have - // access to the value of prompt. + // setTimeout is asynchronous, so the sayHelloInFiveSeconds function will + // exit immediately, and setTimeout will call inner afterwards. However, + // because inner is "closed over" sayHelloInFiveSeconds, inner still has + // access to the 'prompt' variable when it is finally called. } sayHelloInFiveSeconds("Adam") // will open a popup with "Hello, Adam!" in 5s |