The map and set in JS

Using iterable built-in forEach methods

var a = [‘A‘, ‘B‘, ‘C‘];a.forEach(function (element, index, array) { // element: 指向当前元素的值 // index: 指向当前索引 // array: 指向Array对象本身 alert(element);});

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

var s = 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 m = new Map([[1, ‘x‘], [2, ‘y‘], [3, ‘z‘]]);m.forEach(function (value, key, map) { alert(value);});

JS in the parameters do not require consistency, you can need a few write a few (of course, more than write is undefined, and parameter names can casually write "funny")

A.foreach (function (element, index, ARRAY,ADSF) {
Element: A value that points to the current element
Index: point to Current index
Array: points to the array object itself
Console.log (element, index, ARRAY,ADSF);
});
Vm2066:5 a 0 ["a", "B", "C"] undefined
Vm2066:5 B 1 ["A", "B", "C"] undefined
Vm2066:5 C 2 ["A", "B", "C"] undefined

For loop: Similar to foreach in Java:

for (var x of a) {    alert(x); // ‘A‘, ‘B‘, ‘C‘}

The default object representation of JS is {} Key-value pairs, but the keys are all strings, so the map is referenced

var m = new Map ([[' Micheal ', 99],[' Bob ', 90],[' Baobao ', 100]]);

M.get (' micheal ');

You can also do this:

var m = new Map(); // 空Map m.set(‘Adam‘, 67); // 添加新的key-value m.set(‘Bob‘, 59); m.has(‘Adam‘); // 是否存在key ‘Adam‘: true m.get(‘Adam‘); // 67 m.delete(‘Adam‘); // 删除key ‘Adam‘ m.get(‘Adam‘); // undefined


Set is similar to list in Java but the values are not duplicated and the types are changeable;

The Add method adds an element,

Set([1, 2, 3, 3, ‘3‘]);
S.add (4); S.delete (2);

The map and set in JS