TS -- for of循环

# 1 : forEach   --- 循环val值  --- 不可break

var arr = ['a','b','c','d'];

arr.desc = "five";//这里在ts会报错

arr.forEach((val,key) => console.log(val,key));  //a 0   b 1   c 2   d 3

# 2 :for in    --- 循环key键  --- 不忽略属性

var arr = ['a','b','c','d'];

arr.desc = "five";

for( var key in arr){

console.log( key );       //  0,1,2,3,desc     ---  key

}

var arr = ['a','b','c','d'];

arr.desc = "five";

for( var key in arr){

console.log( arr[key] );   //  a,b,c,d,five   --- val

}

# 3：for of    ---  循环val值  --- 忽略属性  --- 可break 

var arr = ['a','b','c','d'];

arr.desc = "five";

for (var val of arr) {

  console.log(val);    // a b c d

}

var arr = [1,2,3,4];

for (var val of arr) {

      if(val>2)  break

       console.log(n);  // 1 2

}