ForEach () map () some () [array expansion method]

2.7.1 Arr.foreach ()

Parameters and Meanings

var arr = [' A ', ' B ', ' C ', ' d ', ' e '];

var RT = Arr.foreach (function (A, B, c) {
console. Log (' Three parameters ', a, b, c);
} ) ;

console. log (' return value ', RT);

The first 1 parameters are each element in the array

The first 2 parameters are indexed

The first 3 parameters are the original array

Characteristics

The 1> ForEach method executes the callback function once for each item in the array that has a valid value in ascending order , those that have been deleted (using the Delete method, and so on) or uninitialized items that are skipped (but do not include those values undefined items)

In the 1> callback function, whatever you return , the end result is undefined .

2.7.2 Arr.map ()

Parameters and Meanings

var arr = [' A ', ' B ', ' C ', ' d ', ' e '];

var RT = Arr.map (function (A, B, c) {
console. Log (' Three parameters ', a, b, c);

} ) ;

console. log (' return value ', RT);

The first 1 parameters are each element in the array

The first 2 parameters are indexed

the 3 parameter is an array of return values for each call to the callback function

Characteristics

The map method invokes the callback function sequentially for each element in the original array . callback The return value (including undefined) after each execution is combined to form a new array. the callback function is only called on the indexed index, and those indexes that have never been assigned a value or deleted by using delete are not called.

Case

Add 2 to each item in the array

var arr = [' A ', ' B ', ' C ', ' d ', ' e '];

var RT = Arr.map (function (item, index, arr) {
console. Log (' Three parameters ', item, index, ARR);
return item + 2;
} ) ;

console. log (' return value ', RT);

2.7.3 Arr.some ()

Parameters and Meanings

var arr = [1, 2, 3, 4, 5];

var RT = Arr.some (function (item, index, arr) {
console. Log (' Three parameters ', item, index, ARR);
return item > 9;
} ) ;

console. log (' return value ', RT);

judging each item in the array, if there is one that satisfies the item > 9 then the some method returns true, otherwise false

Characteristics

Some   callback  callback " " Span style= "font-family: Equal line" > (can be converted to a Boolean true some    true some  return  false callback  has a value " is called on the index , it is not invoked on indexes that have been deleted or have never been assigned a value.

var arr = [1, 2, 3, 4, 5];

var RT = Arr.some (function (item, index, arr) {
console. Log (' Three parameters ', item, index, ARR);
return item > 3;
} ) ;

console. log (' return value ', RT);

ForEach () map () some () [array expansion method]