Array----Map

Map Method (Array) (JavaScript)

Each element of the array invokes the defined callback function and returns an array containing the results.

Grammar

Array1.map (callbackfn[, Thisarg])

Parameters

Parameters	Defined
array1	> must be selected.
callbackfn	> must be selected.    A function that can accept up to three parameters.    map   method will call   callbackfn   function once.
Thisarg	Optional.  The This keyword in the callbackfn function can refer to the object.  if thisargis omitted, undefined is used as the This value.

return value

A new array in which each element is the return value of the callback function for the associated original array element.

Exception

If the callbackfn parameter is not a function object, a TypeError exception is thrown.

Exception	Condition

Notes

For each element in the array, themap method calls the callbackfn function once (in ascending index order).  The callback function will not be called for missing elements in the array.

In addition to array objects, themap method can be used by any object that has a length property and has a property name indexed by number.

callback function Syntax

The syntax for the callback function is as follows:

function Callbackfn (value, index, array1)

You can declare a callback function with up to three parameters.

The following table lists the callback function parameters.

Callback Parameters	Defined
Value	The value of the array element.
Index	The numeric index of the array element.
Array1	The array object that contains the element.

modifying an Array object

An array object can be modified by a callback function.

The following table describes the results obtained from modifying an array object after the map method is started.

Conditions after the map method is started	Is the element passed to the callback function?
Adds an element outside the original length of the array.	Whether.
Add elements to fill the missing elements in the array.	Yes, if the index has not been passed to the callback function.
The element has changed.	Yes, if the element has not been passed to the callback function.
Removes an element from the array.	No, unless the element has been passed to the callback function.

Example

The following example shows the use of the map method.

Javascript

Define the callback function.function areaofcircle (RADIUS) {    var area = Math.PI * (RADIUS * radius);    Return area.tofixed (0);} Create an array.var radii = [Ten, 30];//Get the areas from the radii.var areas = Radii.map (areaofcircle);d OCUMENT.W Rite (areas);//output://314,1257,2827

Example

The following example illustrates the use of the thisarg parameter, which specifies the object that the This keyword can refer to.

Javascript

Define An object this contains a divisor property and//a remainder Function.var obj = {    divisor:10,    remainder : function (value) {        return value% This.divisor;    }}  Create an Array.var numbers = [6, +, 30];//Get the remainders.//the obj argument specifies the this value in the Callback Function.var result = Numbers.map (Obj.remainder, obj);d ocument.write (result);//output://6,2,5,0

Example

In the following example, the built-in JavaScript method is used as a callback function.

Javascript

Apply math.sqrt (value) to all element in an array.var numbers = [9, 16];var result = Numbers.map (math.sqrt);d OCUMENT.W Rite (Result);//output:3,4

Example

The map method can be applied to a string.  This is illustrated in the following example.

Javascript

Define the callback function.function threechars (value, index, str) {    //Create A string that contains the previous , current,    //And next character.    Return str.substring (index-1, index + 2);} Create a String.var word = "Thursday";//Apply The map method to the string.//each of the array element in the result contain s a string that//have the previous, current, and next character.//the commented out statement shows an alternative syntax . var result = [].map.call (Word, threechars);//var result = Array.prototype.map.call (Word, Threechars);d Ocument.write ( result);//output://Th,thu,hur,urs,rsd,sda,day,ay

Array----Map