1.ES6 is the next generation standard of js language and was released in June 2015
2.let const
Let declared variables are only valid in the code block where the let command is located
let can only be declared once
let does not exist. Variable promotion will have a temporary dead zone. If the dead zone is typeof, an error will be reported
const declares a read-only constant. Once declared, the value of the constant cannot be changed. Once declared, it must be initialized, otherwise an error will be reported.
The constant value is the same address. If it is the data in the object array, it can be changed.
That is, if
const arr =[],obj={}
arr.push({obj})//That's OK
arr = new Array[]Just can't, because it will change the address
So is the object
3. Function default value
That's it
function fun (x,y ='111'){
//Here y has a default value
}
4. Deconstruction assignment
let [x,y,z] = [1,2,3]
let {username,pasword} = {username:"11",pasword:'1111'}
If not on the right undefined
let { foo: baz } = { foo: 'aaa', bar: 'bbb' };
baz // "aaa"
foo // error: foo is not defined
//In the above code, foo is the matching pattern and baz is the variable. What is really assigned is the variable baz, not the pattern foo.
5. Extension operator
Three points: expand the array or class array object into a series of values separated by commas
You can merge arrays and objects
6.rest operator
... add a lot of parameters
Is to combine these parameters into an array
rest can only be the last parameter
7. Template string
The array {AAA} contained in curly braces is enclosed in backquotes
8. Arrow function
Features:
Do not bind this value: inherit this from the upper layer of the scope chain
Cannot be used as a constructor
No arguments class array (can be replaced by rest parameter)
Application: is an anonymous function
()=> aaa amount to ()=>{return aaa}
aa=>{} amount to(aaa)=>{}
()=>{}
//This example
var Person = {
'age': 18,
'sayHello': ()=>{
console.log(this.age);
}
};
var age = 20;
Person.sayHello(); // 20
//The scope is inside a function, and there is no function outside the arrow function, so it is global, so it is global. A will output 20. Note that the scope is for the function
9. One of the extensions of array is to convert an array into an array
arguments is not an array and does not inherit array Properties and methods on the prototype object, but with the length attribute,
Convert a class array to an array
Array.prototype.slice.call(arguments)
Array.from() turns the class array into a real array
Array.of() turns a set of values into an array
10. Extension of array prototype method
find() finds the qualified elements in the array. If there are multiple qualified elements, it returns the first one.
let arr = Array.of(1, 2, 3, 4); console.log(arr.find(item => item > 2)); // 3
findIndex() finds the subscript of the qualified element in the array. If there are more than one, it returns the first one.
fill() fills an array element within a range of indexes with a value
let arr = Array.of(1, 2, 3, 4); // Parameter 1: value used to populate // Parameter 2: start index to be filled // Parameter 3 (optional): the end index to be filled. It defaults to the end of the array console.log(arr.fill(0,1,2)); // [1, 0, 3, 4]
copyWithin()
Modifies an array element with a range index to another element with a specified range index for this array.
// Parameter 1: modified start index // Parameter 2: the starting index of the data to be overwritten // Parameter 3 (optional): the end index of the data to be overwritten. The default is the end of the array console.log([1, 2, 3, 4].copyWithin(0,2,4)); // [3, 4, 3, 4] // Parameter 1 is a negative number, indicating the reciprocal console.log([1, 2, 3, 4].copyWithin(-2, 0)); // [1, 2, 1, 2] console.log([1, 2, ,4].copyWithin(0, 2, 4)); // [, 4, , 4]
Extended method
lookup
find()
Find the qualified elements in the array. If there are multiple qualified elements, the first element will be returned.
let arr = Array. of(1, 2, 3, 4); console. log(arr.find(item => item > 2)); // 3 / / array vacancy is processed as undefined console log([, 1].find(n => true)); // undefined
findIndex()
Find the qualified element index in the array. If there are multiple qualified elements, return the first element index.
let arr = Array.of(1, 2, 1, 3); // Parameter 1: callback function / / parameter 2 (optional): specify this value in the callback function console log(arr.findIndex(item => item == 2)); // 1 / / array vacancy is processed as undefined console log([, 1].findIndex(n => true)); // 0
fill
fill()
Populates the contents of array elements with a range of indexes into a single specified value.
let arr = Array.of(1, 2, 3, 4); // Parameter 1: the value to be filled. / / parameter 2: the start index to be filled. / / parameter 3 (optional): the end index to be filled. The default value is console. At the end of the array log(arr.fill(0,1,2)); // [1, 0, 3, 4]
copyWithin()
Modifies an array element with a range index to another element with a specified range index for this array.
//Parameter 1: the modified start index. / / parameter 2: the start index of the data to be overwritten. / / parameter 3 (optional): the end index of the data to be overwritten. The default is console at the end of the array log([1, 2, 3, 4].copyWithin(0,2,4)); // [3, 4, 3, 4] / / parameter 1 is a negative number, indicating the reciprocal console log([1, 2, 3, 4].copyWithin(-2, 0)); // [1, 2, 1, 2] console. log([1, 2,4].copyWithin(0, 2, 4)); // [, 4, 4]
ergodic
entries()
Traverse key value pairs.
for(let [key, value] of ['a', 'b'].entries()){
console.log(key, value);
}
// 0 "a"
// 1 "b"
// Do not use for Of cycle
let entries = ['a', 'b'].entries();
console.log(entries.next().value); // [0, "a"]
console.log(entries.next().value); // [1, "b"]
// Array contains empty bits
console.log([...[,'a'].entries()]); // [[0, undefined], [1, "a"]]
keys()
Traverse key names.
for(let key of ['a', 'b'].keys()){ console.log(key);}// 0 / / 1 / / the array contains empty console log([...[,'a'].keys()]); // [0, 1]
values()
Traverse key values.
contain
includes()
Whether the array contains the specified value.
Nested array to one-dimensional array
flat()
console.log([1 ,[2, 3]].flat()); // [1, 2, 3] / / specify the number of nested layers to convert console log([1, [2, [3, [4, 5]]]].flat(2)); // [1, 2, 3, [4, 5]] / / no matter how many layers of console are nested log([1, [2, [3, [4, 5]]]].flat(Infinity)); // [1, 2, 3, 4, 5] / / automatically skip the empty console log([1, [2, 3]].flat());< p> // [1, 2, 3]
11. Object extension
Concise representation of objects
//Example 1
const foo = 'bar';
const baz = { foo };
baz // {foo: "bar"}
// Equivalent to
const baz = { foo: foo };
//Example 2
const o = {
method() {
return "Hello!";
}
};
// Equivalent to
const o = {
method: function () {
return "Hello!";
}
};
//Example 3
let birth = '2000/01/01';
const Person = {
name: 'Zhang San',
//Equivalent to birth: birth
birth,
// Equivalent to hello: function()
hello() {
console.log('My name is', this.name);
}
};
Attribute name
obj.foo = true
obj['a'+'bc'] =123
let propKey = 'foo'
let obj = {
[propKey]:true,
['a'+'bc']:123
}
Static method
Object.assign(target, source_1, ···)
Used to copy all enumerable properties of the source object to the target object.
let target = {a: 1};
let object2 = {b: 2};
let object3 = {c: 3};
Object.assign(target,object2,object3);
// The first parameter is the target object, and the following parameter is the source object
target; // {a: 1, b: 2, c: 3
Object.is(value1, value2)
It is used to compare whether two values are strictly equal, which is basically similar to (= =).
12.symbol
Here we will talk about data types
Basic data type number string boolean undefined null
The reference data type is Object (Array FUnction date Error)
This symbol is a unique value
var S1 = Symbol()
13.for in traversal
Get key name
14.for of traversal
Get property value
15. Many traversal methods
forEach() is used to traverse the array
arr.forEach((item,index)=>{
})
for in is es5. Traverse the object array to get the key value. It will traverse the prototype attribute
You can use hasOwnPropery to determine whether it is your own attribute
for of is es6 used to traverse the elements in the array
Object.keys () returns an array whose elements are a collection of all key names of the input object
16.Set
Is a constructor that generates a Set through new without duplicate values
17.Map
Similar objects are collections of key value pairs, but the range of keys is not limited to strings. Objects can also be keys