Symbol
A new data type in es6 that represents unique values
Basic data types: String Number Boolean null undefined Symbol
Reference data data type: Object
create:
let s1 = Symbol()
typeof s1 // 'symbol'
Notice:
1. Symbol cannot be called using new because symbol is a primitive type value, not an object.
2. The Symbol function accepts a string as a parameter, which only represents the description of the Symbol. It is mainly used to make it easier to distinguish when displaying on the console or converting to a string.
eg：
let s1 = Symbol()
let s2 = Symbol()
console.log(s1,s2). //Symbol() Symbol()
let s3 = Symbol('js')
let s4 = Symbol('html')
console.log(s3,s4) //Symbol(js) Symbol(html)
console.log(Symbol('html') === Symbol('html')) //false 参数仅仅是Symbol的描述，不代表其他意义
3. If the Symbol parameter is an object, the toString method of the object will be called, converted to a string and then a Symbol value will be generated.
eg：
const obj = {
toString(){
return 'abc'
}
}
console.log(Symbol(obj)) // Symbol(abc)
const obj = {
name: 'aa'
}
console.log(Symbol(obj)) //Symbol([object object])
Symbol type conversion
1.Convert to string
String(symbol('haha'))
2. Convert to Boolean
console.log(!!Symbol()) //true
3. Cannot be converted into numbers and cannot perform any operations.
console.log(Symbol('haha') + 'hello') //报错
4. As the attribute name of the object (because each Symbol value is not equal, this means that Symbol can be used as an identifier for the object)
let s = Symbol('s')
const data = {
[s]: 'haha'
}
console.log(data) //{Symbol(s): 'haha'}
Note: Symbol() as a property name cannot be traversed by for...in, for...of, nor can it be returned by Object.keys(), Object.getOwnPropertyNames(), but it is not a private property and can be obtained through the Object.getOwnPropertySymbols method. All Symbol property names of the object
eg:
let obj = {
},
foo = Symbol('foo')
Object.defineProperty(obj, foo, {
value: 'footer'
})
for(let i in obj) {
console.log(i) //没有返回值
}
console.log(Object.getOwnPropertyNames(obj)) // []
console.log(Object.getOwnPropertySymbols(obj)) [Symbol(foo)]
const data = {
[Symbol()]: 123
}
console.log(data) //{Symbol():123}
console.log(data[Symbol()]) //undefined
console.log(Object.getOwnPropertySymbols(data)) //[Symbol()]
console.log(Object.getOwnPropertySymbols(data)[0]) // 123
Data structureSet
Similar to an array, the values of the members are unique and there are no duplicate values.
1. How to create a Set
const set = new Set()
2.Attributes of Set class
var set = new Set([1,2,3,4])
set.size // 4
3. Methods of the Set class
1) set.add(value): Add a data and return the Set structure itself
set.add('a').add('b').add('c'). //Add multiple values
Or new Set([1,2,3])pass it in directly as a parameter
Note: set does not cast the stored value
set.add(5)
set.add(“5”)
Object.is(new Set().add(5), new Set().add('5')) //false
2) set.delete(value): Delete the specified data and return a Boolean value to indicate whether the deletion is successful.
set.delete(‘a’) // true
set.delete(‘a’). // false
3) set.has(value): Determine whether the value is a Set member and return a Boolean value
console.log(set.has(‘a’)). //false
console.log(set.has(‘b’)). //true
4) set.clear(): clears all data and does not return a value
5) set.keys(): Returns the traverser of key names
const s = new Set([1,2,4])
console.log(s.keys())
6) set.values(): return value traverser
console.log(s.values())
7) set.entries(): returns a traverser of key-value pairs
console.log(set.entries())
8) set.forEach(): Use the callback function to traverse each member
var set = new Set([1,2,4])
set.forEach((value, key, ownerSet) = > {
console.log(`${
key}=${
value}`)
console.log(ownerSet === set). //true
})
Convert set to array:
const set = new Set([1,2,4])
[…set]
Array.from(set)
use:
a. Array deduplication
const arr = [{},1,2,3,1,4,'a',3]
console.log([...new Set(arr)]) //[{},1,2,3,4,'a']
b. Merge and remove duplicates
let a = new Set([1,2,3]);
let b = new Set([3,2,5]);
let union = new Set([...a,...b]);
console.log([...union]); // [1,2,3,5]
Data structureMap
It is an ordered list that stores many key-value pairs. Keys and values support all data types.
Previously: object could only use strings as keys
eg：
let data1 = {a: 1},
data2 = {b: 2},
obj= {}
obj[data1] = 1
obj[data2] = 2
console.log(obj) // {[object Object]: 2}
Map can solve the above problems
var map = new Map().set({}, 1).set({}, 2)
console.log(map) // {Object{}=>1, Object{}=>2}
1.Create Map
const map = new Map()
You can initialize parameters when creating. What is passed is a two-dimensional array. The array inside is in the form of key-value pairs.
const map = new Map([
["a", 1],
["b", 2]
])
console.log(map). // {"a" => 1, "b" => 2 }
2.Map attribute
map.size:The number of key-value pairs in the current data structure
3.Map method
1) set(key, value): Set the key value corresponding to the key name key to value, and return the entire Map structure. If the key already has a value, the key value will be updated, otherwise the key-value pair will be newly generated.
eg:
const map = new Map()
map.set('a',1).set('b',2).set('b',3) /*链式写法*/
console.log(map). //{"a" => 1, "b" => 3}
2.get(key): The get method reads the key value corresponding to the key. If the key cannot be found, undefined is returned.
console.log(map.get('a')) //1
console.log(map.get('c')) //undefined
3.delete(key): Delete a key, return true if the deletion is successful, otherwise return false
console.log(map.delete('a')) // true
console.log(map.delete('d')) // false
4.has(key): The method returns a Boolean value indicating whether a key is in the current Map object.
console.log(map.has('a')) // false
console.log(map.has('b')) // true
5.clear(): clear all data, no return value
map.clear()
map.size //0
6) keys(): Returns the traverser of key names
7) values(): returns a traverser of values
8) entries(): returns a traverser of key-value pairs
9) forEach(): Use the callback function to traverse each member
Use for…of to traverse
const m = new Map()
m.set('a',1).set('b',2).set('c',3)
for(let [key, value] of map){
consoel.log(key,value)
}
Convert map to array:
[…map]
Array.from(map)
Convert map to object:
function strMapTObj(strMap){
let obj = Object.create(null)
for (let [k, v] of strMap){
obj[k] = v
}
return obj
}
const map = new Map().set('yes', true).set('no', false)
strMapTObj(map) // {yes: true, no: false}
Convert object to map:
function objToStrMap(obj) {
let strMap = new Map()
for (let k of Object.keys(obj)){
strMap.set(k, obj[k])
}
return strMap
}
objToStrMap({yes: true, no: false}) // Map('yes'=> true, 'no'=>false)
Some precautions when using Map
const map = new Map()
map.set(NaN, 10).set(NaN, 100). //{NaN => 100}