js for 循环跳过undefined值

var narr=['a','b','c','d','e'];
console.log(narr);
for (var j=0;j<narr.length;j++){
   console.log(j+'=>'+narr[j]);
};

console.log(narr);
for (var j=0;j<narr.length;j++){
   console.log(j+'=>'+narr[j]);
   if (narr[j] == 'c'){
         //narr.splice(j,1);
         delete narr[j];
   };
};
console.log(narr);
for (var j in narr){
   console.log(j+'=>'+narr[j]);
};



(5) ["a", "b", "c", "d", "e"]0: "a"1: "b"3: "d"4: "e"length: 5__proto__: Array(0)
h23.js:4 0=>a
h23.js:4 1=>b
h23.js:4 2=>c
h23.js:4 3=>d
h23.js:4 4=>e
h23.js:7 (5) ["a", "b", "c", "d", "e"]0: "a"1: "b"3: "d"4: "e"length: 5__proto__: Array(0)
h23.js:9 0=>a
h23.js:9 1=>b
h23.js:9 2=>c
h23.js:9 3=>d
h23.js:9 4=>e
h23.js:15 (5) ["a", "b", empty, "d", "e"]0: "a"1: "b"3: "d"4: "e"length: 5__proto__: Array(0)
h23.js:17 0=>a
h23.js:17 1=>b
h23.js:17 3=>d
h23.js:17 4=>e


这种方式遍历跳过undefined值