const声明常量，只读、值不能改变。
const PI = 3.1415; PI = 3; // Uncaught TypeError: Assignment to constant variable.
声明必须赋值
const PI //Uncaught SyntaxError: Missing initializer in const declaration
有块级作用域
if (true) {
const PI = 5;
}
PI // Uncaught ReferenceError: PI is not defined
没有变量提升
if (true) {
console.log(PI); //Uncaught ReferenceError: Cannot access ‘PI‘ before initialization
const PI = 3;
}
不可重复声明
const a = 1; const a = 2; // Uncaught SyntaxError: Identifier ‘a‘ has already been declared
原文：https://www.cnblogs.com/blogZhao/p/12517073.html