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New data structure Map

The essence of an object in JavaScript is a collection of key-value pairs, but keys can only be strings. In order to make up for this deficiency, ES6 brings a new data structure, Map. Map is also a collection of key-value pairs, except that keys can be not only strings but also other data types, such as objects (isn't that amazing). Take a look at the following example.

Var m = new Map (); var ul = document.getElementsByTagName ('ul'); m.set (ul,'hi'); console.log (m.get (ul)); / / hi

There are several operations for Map:

M.set (ul,'content'); / / add member m.get (ul) to Map; / / get the value m.has (ul) corresponding to key ul; / / return Boolean value to determine whether there is a key ulm.delete (ul); / / delete key ul, return true for success, return falsem.size / / return m-length m.clear () for failure; / / clear all m members

The method of assigning an initial value to Map directly:

Var m = new Map ([[li_1,'hello'], [li_2,'world']])

The accepted parameter is an array, and the members of the array are an array that represents a key-value pair. If you are confused, take a look at the actual execution of the uplink code:

Var li_1 = document.getElementsByTagName ('li') [0]; var li_2 = document.getElementsByTagName (' li') [1]; var list = [[li_1,'hello'], [li_2,'world']]; var m = new Map (); list.forEach (([key,value]) = > m.set (key,value)); console.log (m.get (li_1)); / / hello

If you assign a value to a key multiple times, the subsequent value overrides the previous value. It is worth noting that when the key is an object, the reference must be the same before the key is considered to be the same. The traversal method of Map is described below.

Var ul = document.getElementsByTagName ('ul'); var li_1 = document.getElementsByTagName (' li') [0]; var li_2 = document.getElementsByTagName ('li') [1]; var list = [li_1,'hello'], [li_2,'world']]; var m = new Map (); list.forEach (([key,value])) = > m.set (key,value); for (let key of m.keys ()) {console.log (key) } for (let val of m.values ()) {console.log (val);} for (let item of m.entries ()) {console.log (item [0], item [1]);} for (let [key,val] of m.entries ()) {console.log (key,val);} m.forEach (function (val, key, ul) {console.log (this); / / ul console.log (val, key);}, ul) / / the second parameter of forEach is used to change the this point in the anonymous function.

Conversion between Map and other data types

The easiest way for Map to transfer data is to use... Extension operator. For example:

Console.log (... m); / / [li, "hello"] [li, "world"]

When Map converts an object, all keys must be strings, using the Object.create () function. When Map transforms to JSON, it also requires that all keys are strings, using the JSON.Stringify () function.

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