javascript memo: function

<br />//javascript 也可以這樣用<br />a = a || []<br /><br />for(var property in o) a.push(property)<br />

Arguments
<br />function f(x, y, z) { <br /> // First, verify that the right number of arguments was passed <br /> if (arguments.length != 3) { <br /> throw new Error("function f called with " + arguments.length + <br /> "arguments, but it expects 3 arguments."); <br /> } <br /> // Now do the actual function... <br />} <br /><br />// built-in function Math.max(), which behaves the same way<br />function max(/* ... */) { <br /> var m = Number.NEGATIVE_INFINITY; <br /> // Loop through all the arguments, <br /> looking for, and <br /> // remembering, the biggest <br /> for(var i = 0; i < arguments.length; i++) <br /> if (arguments[i] > m) m = arguments[i]; <br /> // Return the biggest <br /> return m; <br />} <br />var largest = max(1, 10, 100, 2, 3, 1000, 4, 5, 10000, 6);<br />
callee:
The Arguments object defines a callee property that refers to the function that is currently being executed.
<br />function(x) { if (x <= 1) return 1; <br /> return x * arguments.callee(x-1); <br />} <br />
Using Object Properties as Arguments
<br />function easycopy(args) { arraycopy(args.from, <br /> args.from_start || 0, // Note default value provided <br /> args.to, <br /> args.to_start || 0, <br /> args.length); <br />} <br />// Here is how you might invoke easycopy(): <br />var a = [1,2,3,4]; <br />var b = new Array(4); <br />easycopy({from: a, to: b, length: 4}); <br />
instanceof和typeof
<br />// Return the sum of the elements of array (or array-like object) a. // The elements of a must all be numbers, but null and undefined <br />// elements are ignored. <br />function sum(a) { <br /> if ((a instanceof Array) || // if array <br /> (a && typeof a == "object" && "length" in a)) { // or array like <br /> var total = 0; <br /> for(var i = 0; i < a.length; i++) { <br /> var element = a[i]; <br /> if (!element) continue; // ignore null and undefined elements <br /> if (typeof element == "number") total += element; <br /> else throw new Error("sum(): all array elements must be numbers"); <br /> } <br /> return total; <br /> } <br /> else throw new Error("sum(): argument must be an array"); <br />} <br />
<br />function check(args) { <br /> var actual = args.length; // The actual number of arguments <br /> var expected = args.callee.length; // The expected number of arguments , 這個就是Function.length<br /> if (actual != expected) { // Throw an exception if they don't match <br /> throw new Error("Wrong number of arguments: expected: " + <br /> expected + "; actually passed " + actual); <br /> } <br />} <br />function f(x, y, z) { <br /> // Check that the actual # of args matches the expected # of args <br /> // Throw an exception if they don't match <br /> check(arguments); <br /> // Now do the rest of the function normally <br /> return x + y + z; <br />}<br />
apply and call
<br />f.call(o, 1, 2); <br />//This is similar to the following lines of code: <br />o.m = f; <br />o.m(1,2); <br />delete o.m; <br /><br />//The apply() method is like the call() method, except that the arguments to be passed to the function are specified as an array: <br />f.apply(o, [1,2]);<br />
The Function() Constructor
<br />//A last, very important point about the Function() constructor is that the functions it creates do not use lexical scoping; instead, they are always compiled as if they were top-level functions, as the following code demonstrates:<br />var y = "global"; <br />function constructFunction() { <br /> var y = "local"; <br /> return new Function("return y"); // Does not capture the local scope! <br />} <br />// This line displays "global" because the function returned by the <br />// Function() constructor does not use the local scope. Had a function <br />// literal been used instead, this line would have displayed "local". <br />alert(constructFunction()()); // Displays "global" <br />