Introduction to ES6 (1)

Introduction to ES6 (1)

Block-level scope-let command, const command

-Variables declared with the let command will not be promoted

(function(){
    if(true){
        let a = 5;
    }
    console.log(a); //报错 
})

-Variables declared with the const command must be assigned at the time of declaration

const a; //报错

-If a variable declared with the const command is assigned to an object, the address it points to cannot be changed, and the properties of the object cannot be controlled.

const a = {};
a.name = 'Tom';//正确
a = {name: 'Tom'};//报错

If you need to freeze the object, you need to use the Object.freeze method

var obj = Object.freeze({});
obj.age = 18;//常规模式下无效，严格模式下报错

To freeze the object itself and the properties of the object, you can use the following method

var constantize = (obj) => {
  Object.freeze(obj);
  Object.keys(obj).forEach( (key, i) => {
    if ( typeof obj[key] === 'object' ) {
      constantize( obj[key] );
    }
  });
};

-ES6 Six methods of declaring variables: var command, function command, let command, const command, import command, class command

-Method to get the top-level object of the page

// 方法一
(typeof window !== 'undefined'
   ? window
   : (typeof process === 'object' &&
      typeof require === 'function' &&
      typeof global === 'object')
     ? global
     : this);

// 方法二
var getGlobal = function () {
  if (typeof self !== 'undefined') { return self; }
  if (typeof window !== 'undefined') { return window; }
  if (typeof global !== 'undefined') { return global; }
  throw new Error('unable to locate global object');
};