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一個函式的定義由一系列的函式關鍵詞組成, 依次為:

  •     函式的名稱。
  •     包圍在括號()中,並由逗號區隔的一個函式參數列表。
  •     包圍在大括號{}中,用於定義函式功能的一些JavaScript語句。



function square(number) {
  return number * number;

函式square有一個參數,叫作number。這個函式只有一行程式碼,它會回傳number自乘的結果。函式return 語法描述函式的返回值。

return number * number;


如果傳遞一個物件(例如 Array 或自定義的其它物件)作為參數,而函式改變了這個物件的屬性,這樣的改變對函式外部是有作用的(因為是傳遞物件的位址),如下面的例子所示

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

var mycar = {make: "Honda", model: "Accord", year: 1998},

x = mycar.make;     // x 的值為 "Honda"

y = mycar.make;     // y 的值為 "Toyota"
                    // (屬性 make 被 function 改變)


function myFunc(theObject) {
  theObject = {make: "Ford", model: "Focus", year: 2006};

var mycar = {make: "Honda", model: "Accord", year: 1998},

x = mycar.make;     // x 的值為 "Honda"

y = mycar.make;     // y 的值還是 "Honda" 


var square = function(number) {return number * number};
var x = square(4) //x 的值為 16
var factorial = function fac(n) {return n<2 ? 1 : n*fac(n-1)};



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


map(function(x) {return x * x * x}, [0, 1, 2, 5, 10]);
// 結果會回傳 [0, 1, 8, 125, 1000]

除了上述的定義方式以外,我們也可以透過 Function constructor 來定義, 類似 eval().





Functions must be in scope when they are called, but the function declaration can be below the call, as in this example:

/* ... */
function square(n){return n*n} 

The scope of a function is the function in which it is declared, or the entire program if it is declared at the top level. Note that this works only when defining the function using the above syntax (i.e. function funcName(){}). The code below will not work.

square = function (n) {
  return n * n;

The arguments of a function are not limited to strings and numbers. You can pass whole objects to a function, too. The show_props function (defined in Working with Objects) is an example of a function that takes an object as an argument.

A function can be recursive; that is, it can call itself. For example, here is a function that computes factorials recursively:

function factorial(n){
  if ((n == 0) || (n == 1))
    return 1;
    return (n * factorial(n - 1));

You could then compute the factorials of one through five as follows:

var a, b, c, d, e;
a = factorial(1); // a gets the value 1
b = factorial(2); // b gets the value 2
c = factorial(3); // c gets the value 6
d = factorial(4); // d gets the value 24
e = factorial(5); // e gets the value 120

There are other ways to call functions. There are often cases where a function needs to be called dynamically, or the number of arguments to a function vary, or in which the context of the function call needs to be set to a specific object determined at runtime. It turns out that functions are, themselves, objects, and these objects in turn have methods (see the Function object). One of these, the apply() method, can be used to achieve this goal.

Function scope

Variables defined inside a function cannot be accessed from anywhere outside the function, because the variable is defined only in the scope of the function. However, a function can access all variables and functions defined inside the scope in which it is defined. In other words, a function defined in the global scope can access all variables defined in the global scope. A function defined inside another function can also access all variables defined in it's parent function and any other variable to which the parent function has access.

// The following variables are defined in the global scope
var num1 = 20,
    num2 = 3,
    name = "Chamahk";

// This function is defined in the global scope
function multiply() {
  return num1 * num2;

multiply(); // Returns 60

// A nested function example
function getScore () {
  var num1 = 2,
      num2 = 3;
  function add() {
    return name + " scored " + (num1 + num2);
  return add();

getScore(); // Returns "Chamahk scored 5"


Closures are one of the most powerful features of JavaScript. JavaScript allows for the nesting of functions and, in addition, grants the inner function full access to all the variables and functions defined inside the outer function (and all other variables and functions that the outer function has access to). However, the outer function does not have access to the variables and functions defined inside the inner function. This provides a sort of security for the variables of the inner function. Also, since the inner function has access to the scope of the outer function, the variables and functions defined in the outer function will live longer than the outer function itself, if the inner function manages to survive beyond the life of the outer function. A closure is created when the inner function is somehow made available to any scope outside the outer function.

var pet = function(name) {          // The outer function defines a variable called "name"
      var getName = function() {
        return name;                // The inner function has access to the "name" variable of the outer function

      return getName;               // Return the inner function, thereby exposing it to outer scopes
    myPet = pet("Vivie");
myPet();                            // Returns "Vivie"

It can be much more complex than the code above. An object containing methods for manipulating the inner variables of the outer function can be returned.

var createPet = function(name) {
  var sex;
  return {
    setName: function(newName) {
      name = newName;
    getName: function() {
      return name;
    getSex: function() {
      return sex;
    setSex: function(newSex) {
      if(typeof newSex == "string" && (newSex.toLowerCase() == "male" || newSex.toLowerCase() == "female")) {
        sex = newSex;

var pet = createPet("Vivie");
pet.getName();                  // Vivie

pet.getSex();                   // male
pet.getName();                  // Oliver

In the codes above, the name variable of the outer function is accessible to the inner functions, and there is no other way to access the inner variables except through the inner functions. The inner variables of the inner function act as safe stores for the inner functions. They hold "persistent", yet secure, data for the inner functions to work with. The functions do not even have to be assigned to a variable, or have a name.

var getCode = (function(){
  var secureCode = "0]Eal(eh&2";    // A code we do not want outsiders to be able to modify...
  return function () {
    return secureCode;

getCode();    // Returns the secret code

There are, however, a number of pitfalls to watch out for when using closures. If an enclosed function defines a variable with the same name as the name of a variable in the outer scope, there is no way to refer to the variable in the outer scope again.

var createPet = function(name) {  // Outer function defines a variable called "name"
  return {
    setName: function(name) {    // Enclosed function also defines a variable called "name"
      name = name;               // ??? How do we access the "name" defined by the outer function ???

The magical this variable is very tricky in closures. They have to be used carefully, as what this refers to depends completely on where the function was called, rather than where it was defined. An excellent and elaborate article on closures can be found here.

Using the arguments object

The arguments of a function are maintained in an array-like object. Within a function, you can address the arguments passed to it as follows:


where i is the ordinal number of the argument, starting at zero. So, the first argument passed to a function would be arguments[0]. The total number of arguments is indicated by arguments.length.

Using the arguments object, you can call a function with more arguments than it is formally declared to accept. This is often useful if you don't know in advance how many arguments will be passed to the function. You can use arguments.length to determine the number of arguments actually passed to the function, and then access each argument using the arguments object.

For example, consider a function that concatenates several strings. The only formal argument for the function is a string that specifies the characters that separate the items to concatenate. The function is defined as follows:

function myConcat(separator) {
   var result = "", // initialize list
   // iterate through arguments
   for (i = 1; i < arguments.length; i++) {
      result += arguments[i] + separator;
   return result;

You can pass any number of arguments to this function, and it concatenates each argument into a string "list":

// returns "red, orange, blue, "
myConcat(", ", "red", "orange", "blue");

// returns "elephant; giraffe; lion; cheetah; "
myConcat("; ", "elephant", "giraffe", "lion", "cheetah");

// returns "sage. basil. oregano. pepper. parsley. "
myConcat(". ", "sage", "basil", "oregano", "pepper", "parsley");

Please note that the arguments variable is "array-like", but not an array. It is array-like in that is has a numbered index and a length property. However, it does not possess all of the array-manipulation methods.

See the Function object in the JavaScript Reference for more information.

Predefined functions

JavaScript has several top-level predefined functions:

The following sections introduce these functions. See the JavaScript Reference for detailed information on all of these functions.

eval Function

The eval function evaluates a string of JavaScript code without reference to a particular object. The syntax of eval is:


where expr is a string to be evaluated.

If the string represents an expression, eval evaluates the expression. If the argument represents one or more JavaScript statements, eval performs the statements. The scope of eval code is identical to the scope of the calling code. Do not call eval to evaluate an arithmetic expression; JavaScript evaluates arithmetic expressions automatically.

isFinite function

The isFinite function evaluates an argument to determine whether it is a finite number. The syntax of isFinite is:


where number is the number to evaluate.

If the argument is NaN, positive infinity or negative infinity, this method returns false, otherwise it returns true.

The following code checks client input to determine whether it is a finite number.

   /* take specific steps */

isNaN function

The isNaN function evaluates an argument to determine if it is "NaN" (not a number). The syntax of isNaN is:


where testValue is the value you want to evaluate.

The parseFloat and parseInt functions return "NaN" when they evaluate a value that is not a number. isNaN returns true if passed "NaN," and false otherwise.

The following code evaluates floatValue to determine if it is a number and then calls a procedure accordingly:

var floatValue = parseFloat(toFloat);

if (isNaN(floatValue)) {
} else {

parseInt and parseFloat functions

The two "parse" functions, parseInt and parseFloat, return a numeric value when given a string as an argument.

The syntax of parseFloat is:


where parseFloat parses its argument, the string str, and attempts to return a floating-point number. If it encounters a character other than a sign (+ or -), a numeral (0-9), a decimal point, or an exponent, then it returns the value up to that point and ignores that character and all succeeding characters. If the first character cannot be converted to a number, it returns "NaN" (not a number).

The syntax of parseInt is:

parseInt(str [, radix]);

parseInt parses its first argument, the string str, and attempts to return an integer of the specified radix (base), indicated by the second, optional argument, radix. For example, a radix of ten indicates to convert to a decimal number, eight octal, sixteen hexadecimal, and so on. For radixes above ten, the letters of the alphabet indicate numerals greater than nine. For example, for hexadecimal numbers (base 16), A through F are used.

If parseInt encounters a character that is not a numeral in the specified radix, it ignores it and all succeeding characters and returns the integer value parsed up to that point. If the first character cannot be converted to a number in the specified radix, it returns "NaN." The parseInt function truncates the string to integer values.

Number and String functions

The Number and String functions let you convert an object to a number or a string. The syntax of these functions is:

var objRef;
objRef = Number(objRef);
objRef = String(objRef);

where objRef is an object reference. Number uses the valueOf() method of the object; String uses the toString() method of the object.

The following example converts the Date object to a readable string.

var D = new Date(430054663215),
x = String(D); // x equals "Thu Aug 18 04:37:43 GMT-0700 (Pacific Daylight Time) 1983"

The following example converts the String object to Number object.

var str = "12",
num = Number(str);

You can check it. Use DOM method write() and JavaScript typeof operator.

var str = "12",
document.write(typeof str);
num = Number(str);
document.write(typeof num);

escape and unescape functions(Obsoleted above JavaScript 1.5)

The escape and unescape functions do not work properly for non-ASCII characters and have been deprecated. In JavaScript 1.5 and later, use encodeURI, decodeURI, encodeURIComponent, and decodeURIComponent.

The escape and unescape functions let you encode and decode strings. The escape function returns the hexadecimal encoding of an argument in the ISO Latin character set. The unescape function returns the ASCII string for the specified hexadecimal encoding value.

The syntax of these functions is:


These functions are used primarily with server-side JavaScript to encode and decode name/value pairs in URLs.


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