Functions

本文需要你對javascript稍微有一點了解，最最基礎的東西本文略過不講。

本文不是來自于本人的經驗，而是來自于https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Functions

今天本人沒什么事，給大家翻譯翻譯，英文好的自己去看原版。

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Primitive parameters (such as a number) are passed to functions by value; the value is passed to the function, but if the function changes the value of the parameter, this change is not reflected globally or in the calling function。

1 如果你傳一個簡單參數給function，比如說var number=3 ，那么即使在function內部改變了這個number的值，改變也不會反應到function的外部。

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If you pass an object (i.e. a non-primitive value, such as Array or a user-defined object) as a parameter, and the function changes the object's properties, that change is visible outside the function

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2 如果你傳一個對象，比如說一個數組，并且function內部改變了數組的屬性，那么這個變化就會反應到function的外部了。如下面的例子。

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function myFunc(theObject) {
? theObject.make = "Toyota";
}

var mycar = {make: "Honda", model: "Accord", year: 1998},
??? x,
??? y;

x = mycar.make;??? // x gets the value "Honda"

myFunc(mycar);
y = mycar.make;??? // y gets the value "Toyota" ?????????????????? // 函數myFunc改變了make屬性

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Note that assigning a new object to the parameter will not have any effect outside the function, because this is changing the value of the parameter rather than the value of one of the object's properties:

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3 值得注意的是，如果在function內部重新給對象賦值，是不會影響function外部的，因為這個情況下javascript對待這個傳入的對象和對待簡單參數的方式是一致的。給對象重新賦值就好像改變了number的值一樣。

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function myFunc(theObject) {theObject = {make: "Ford", model: "Focus", year: 2006}; }var mycar = {make: "Honda", model: "Accord", year: 1998},x,y;x = mycar.make; // x gets the value "Honda" myFunc(mycar); y = mycar.make; // y still gets the value "Honda"

In the second case, the function did not alter the object that was passed; instead, it created a new local variable that happens to have the same name as the global object passed in, so there is no effect on the global object that was passed in.

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在第二個例子中，function內部創建了一個新的局部變量，并且改變量恰好和全局的對象重名，所以對這個全局對象沒有影響。

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While the function declaration above is syntactically a statement, functions can also be created by a?function expression. Such a function can be?anonymous; it does not have to have a name.

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定義function還有另外一種寫法，叫做函數表達式。這樣的function可以是匿名的。沒有必要非寫一個名字給function。如下面的例子：

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var square = function(number) {return number * number};
var x = square(4) //x gets the value 16

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However, a name can be provided with a function expression, and can be used inside the function to refer to itself, or in a debugger to identify the function in stack traces:

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當然函數表達式也可以提供名字，這個名字可以用來在function內部調用自己，或者調試的時候用到。如下面的例子：

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var factorial = function fac(n) {return n<2 ? 1 : n*fac(n-1)};

print(factorial(3));

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Function expressions are convenient when passing a function as an argument to another function. The following example shows a?map?function being defined and then called with an anonymous function as its first parameter:

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函數表達式可以很方便的把一個function當參數傳遞給另一個function，如下面的例子，第一個參數就是一個匿名函數。

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function map(f,a) {
? var result = [], // Create a new Array
????? i;
? for (i = 0; i != a.length; i++)
??? result[i] = f(a[i]);
? return result;
}

調用的時候：

map(function(x) {return x * x * x}, [0, 1, 2, 5, 10]); 結果：?

[0, 1, 8, 125, 1000].

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In JavaScript, a function can be defined based on a condition. For example, the following function definition defines myFunc only if num equals 0:

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在javascript中，function定義可以在寫在判斷中，下面的例子，myFunc只有num=0時才被定義。

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var myFunc;
if (num == 0){
? myFunc = function(theObject) {
??? theObject.make = "Toyota"
? }
}

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In addition to defining functions as described here, you can also use the Function constructor to create functions from a string at runtime, much like eval().

A method is a function that is a property of an object. Read more about objects and methods in Working with Objects.

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另外還可以用function構造器在運行時創建function，很像eval（）的方式。method是一個函數并且該函數是一個對象的屬性，更多的內容請閱讀 working with object 。 ?

working with object是我的另一篇博客javascript 之 function2

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未完待續。。。

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轉載于:https://www.cnblogs.com/hmdrzql/p/3482118.html

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