# Variables

# var

• The var keyword assigns a function scope to the variable
• Variables declared using var do not support block level scope
  • if a variable is declared in a loop or if block it can be accessed outside the loop or the if block
  • var keyword support hoisting(状态提升)
  • 变量可以在声明前使用，因为提升到了作用域的顶端
• If we declare the same variable twice using the var keyword within a block, the compiler will not throw an error

# let

• ES6 defines a new variable scope —— The Block scope
• allows the script to restrict access to the variable to the nearest enclosing { block }
• let can't be re-declared

  function test() { 
   var num = 100;
   console.log(num); // 100
  //  {  let as block
      let num = 200 
      console.log(num)  // 200
  //  } 
} 
test();

# const

• const declaration creates a read-only reference to a value
• It does not mean the value it holds is immutable, just that the variable identifier cannot be reassigned
• 其实最好用就是destructuring (解构)啦

   const {
          book: { name: bkname  },//取对象的内部因为重名就起个别名
          source: { name: sorname = {}} // apply a default to avoid undefined
        } = row
      console.log(bkname,sourname)

• "constants" variable are immutable
• reinitializing [array] will result in an error, but arr.push(..) is still ok ~