04.javascript--Introduction Some methods record the iterable

Traversal Array can take the subscript loop, traverse Map and Set cannot use subscript. In order to unify collection types, the ES6 standard introduces new iterable types Array , Map and Set all of them belong to iterable types.

A iterable collection with a type can traverse through a new for ... of loop.

varA = [' A ', ' B ', ' C '];vars =NewSet ([' A ', ' B ', ' C ']);varm =NewMap ([[1, ' X '], [2, ' Y '], [3, ' Z ']]); for(varX of a) {//Traversal Arrayalert (x);} for(varX of s) {//traversing setalert (x);} for(varX of M) {//Traverse MapAlert (x[0] + ' = ' + x[1]);}

You may have questions about for ... of for ... in What is the difference between loops and loops?

for ... inThe loop is actually a property name of the object that is traversed because of historical problems. An Array array is actually an object, and its index of each element is treated as an attribute.

When we manually Array add extra attributes to an object, the for ... in loop brings unexpected unintended effects:

var a = [' A ', ' B ', ' C '= ' Hello ';  for (var in a) {    //  ' 0 ', ' 1 ', ' 2 ', ' Name '}

for ... inLoops will be name included, but Array the length attributes are not included.

for ... ofLoops completely fix these problems by looping only the elements of the collection itself:

var a = [' A ', ' B ', ' C '= ' Hello ';  for (var  X of a) {    //  ' A ', ' B ', ' C '}

This is why you should introduce a new for ... of loop.

However, a better approach is to use the built-in iterable forEach method directly, which receives a function that automatically callbacks the function each time it iterates. Take Array for example:

var a = [' A ', ' B ', ' C '];a.foreach (  element, index, array) {    //  element: A value pointing to the current element    //  Index: Pointing to the current index    //  array: Pointing to the Array object itself      alert (element);});

SetArraysimilar but Set no index, so the first two parameters of the callback function are the elements themselves:

var New Set ([' A ', ' B ', ' C ']); S.foreach (function  (element, sameelement, set) {    alert (Element) ;});

MapThe callback function parameters are sequentially value , key and map themselves:

var New Map ([[1, ' X '], [2, ' Y '], [3, ' Z ']]); M.foreach (function  (value, key, map) {    alert (value);});

04.javascript--Introduction Some methods record the iterable