ES6 new grammar

Why is ES6

Every birth criteria are meant to strengthen the language of sound, function, JavaScript language itself, there are some places unsatisfactory

• Variable upgrade feature adds unpredictability program is running
• The syntax is too loose, to achieve the same functionality, different people might write different code

ES6 new grammar

let

ES6 the new keyword is used to declare variables

• Variables declared valid only let located block level
• To prevent the loop variable becomes a global variable
• There is no variable lift
• Block-level temporary area where the dead and let bind

NOTE: Use only let keyword variable declared with block-level scope, a variable declared with var block-level scope does not have the characteristic

const

Role: declare a constant, constant values ​​can not be changed is the value (memory address)

• Having a block-level scope
• When the assignment must declare constants
• When constant values, the value can not be changed (can not change the memory address)

let const var difference

1. var scoped within the function where the play of the statement, and there is a variable phenomenon upgrade
2. let the scope of the statements of the block where the variable lift absent
3. const appears later in the code can not modify the value of this constant

Deconstruction assignment

• S6, allows the extraction of values ​​from the array, in a corresponding position, variable assignment, the object can be achieved deconstructed
• According to a certain pattern, or an array of values ​​from the extracted object, the value assigned to the extracted additional variables

An array of deconstruction

• Array structure allows us to-one relationship extract value from the array and the value assigned to the variable

One correspondence, if no value is undefind

Object deconstruction

Arrow function

() => {}

To simplify the function definition syntax

• Only the results of a function in the code, and the code is returned, one can omit the braces

• If the function parameter is only one parameter is outside the parentheses may be omitted

function fn(v) {
    return v;
}
const fn = v => v;

this points to a problem

Arrow function does not bind this keyword, the function of this arrow, points to the context of this function defines the location

The remaining parameters

The remaining parameter syntax allows us to a variable number of arguments is represented as an array

const sum = (...args) => {
            let total = 0;
            args.forEach(item =>  total += item)
            return total;
        }
        console.log(sum(10,20)); //30
        console.log(sum(10,20,30)); //60

The remaining parameters and with the use of deconstruction

 let ary = ['zhangsan','lisi','wangwu'];
        let [s1,...s2] = ary;
        console.log(s1); //'zhangsan'
        console,log(s2);//'zhangsan', 'lisi'