How to use foreach() in es6

In es6, the foreach() method is used to traverse the array, call each element of the array, and pass the element to the callback function for processing, the syntax "array.forEach(function(currentValue,index,arr) ,thisValue)”.

The operating environment of this tutorial: Windows 7 system, ECMAScript version 6, Dell G3 computer.

In es6, the foreach() method is used to traverse the array, call each element of the array, and pass the element to the callback function for processing. The specific description is as follows:

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

Parameter	Description
function(currentValue, index, arr )	Required. The function that needs to be called for each element in the array. Function parameters: Parameter description currentValue is required. The current element index is optional. The index value of the current element. arr is optional. The array object to which the current element belongs.
thisValue	Optional. The value passed to the function usually uses the "this" value. If this parameter is empty, "undefined" will be passed to the "this" value

For each element that appears in the array, the forEach method will call the callbackfn function once , in ascending index order, but the callback function is not called for empty elements in the array.

In addition to array objects, the forEach method can also be used on any object that has a length property and has a numerically indexed property name, such as associative array objects, Arguments, etc.

The forEach 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 forEach method is started are as shown in the table.

The impact of the callback function modifying the array

Conditions after the forEach 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 forEach to iterate the array a, and then output and display the value and subscript index of each element. The code is as follows:

function f(value,index,array) {
    console.log("a[" + index + "] = " + value);
}
var a = ['a', 'b', 'c'];
a.forEach(f);

The demonstration results are as follows:

Example 2

The following example uses forEach to iterate the array a, then calculate the sum of the array elements and output it.

var a = [10, 11, 12], sum = 0;
a.forEach (function (value) {
    sum += value;
});
console.log(sum);  //返回33

Example 3

The following example demonstrates how to use the second parameter of the forEach() method, which is the this passing object of the callback function. When iterating over an array, the value of the array element is first read and then its value is overwritten.

var obj = {
    f1 : function(value,index,array) {
        console.log("a[" + index + "] = " + value);
        arrar[index] = this.f2(value);
    },
    f2 : function (x) {return x * x;}
};
var a = [12, 26, 36];
a.forEach(obj.f1, obj);
console.log(a);  //返回[144, 676, 1296]

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