What is the use of the map() method of arrays in ecmascript 5?

In ecmascript, the map() method can call the specified callback function for processing on each element of the array, and return an array containing the results; syntax "array.map(function(currentValue,index,arr) , thisValue)".

The operating environment of this tutorial: windows7 system, javascript version 1.8.5, Dell G3 computer.

The map() method can call the specified callback function for each element of the array and return an array containing the results.

The map() method processes elements sequentially in the order of the original array elements.

Syntax

array.map(function(currentValue,index,arr), thisValue)

Parameters	Description
function(currentValue, index,arr)	Required. Function, each element in the array will execute this function. Function parameters: currentValue is required. The value of the current element #index is optional. The index value of the current element #arr Optional. The array object to which the current element belongs
thisValue	Optional. The object is used as the execution callback, passed to the function, and used as the value of "this". If thisValue is omitted, or null or undefined is passed in, then the this of the callback function is the global object.

The map() method will return a new array, where each element is the callback function return value of the associated original array element. For each element in the array, the map() method calls the callbackfn function once (in ascending index order) and does not call the callback function for missing elements in the array.

In addition to array objects, the map() method can be used by any object that has a length property and has an indexed property name, such as an Arguments parameter object.

The map() method does not directly modify the original array, but the callback function may modify it. The results obtained by modifying the array object after the map method is started are shown in the table.

The impact of the callback function modifying the array

Conditions after the map method is started	Whether the element is passed to the callback function
Add elements beyond the original length of the array	No
Add elements to fill 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
Remove element from array	No unless the element has been passed to the callback function

Example 1

The following example uses the map() method to map an array, square the value of each element in the array, multiply it by the PI value, use the area value of the returned circle as the element value of the new array, and finally return the new array.

function f (radius) {
    var area = Math.PI * (radius * radius);
    return area.toFixed(0);
}
var a = [10,20,30];
var a1 = a.map(f);
console.log(a1);

Example 2

The following example uses the map() method to map an array and divide the value of each element in the array by A threshold, and then returns this new array where both the callback function and the threshold exist as properties of the object. This method demonstrates how to use the thisArg parameter in the map.

var obj = {
    val : 10,
    f : function (value) {
        return value % this.val;
    }
}
var a = [6,12,25,30];
var a1 = a.map(obj.f, obj);
console.log(a1);  //6,2,5,0

Example 3

The following example demonstrates how to use JavaScript built-in methods as callback functions.

var a = [9, 16];
var a1 = a.map(Math.sqrt);
console.log(a1);  //3,4

Example 4

The following example demonstrates how to use the map method to apply to a class array. In the example, map is applied to a string through the dynamic calling method (Call). Then map will traverse each character in the string and call the callback function threeChars to intercept the three characters to the left and right of each character and map it to in a new array.

function f (value, index, str) {
    return str.substring(index - 1, index + 2);
}
var s = "Thursday";
var a = [].map.call(s, f);
console.log(a);  //Th,Thu,hur,urs,rsd,sda,day,ay

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