let strArr = ['a', 'b', 'c', 'd'],
  i = 0,
  length = strArr.length;
for (; i < length; i++) {
  console.log(strArr[i]);//a
  if(arr[i] === 'a'){
    //do something
  };
  break;
};

try {
  var arr = [1, 2, 3, 4];
  arr.forEach(function (item, index) {
    //跳出条件
    if (item === 3) {
      throw new Error("LoopTerminates");
    }
    //do something
    console.log(item);
  });
} catch (e) {
  if (e.message !== "LoopTerminates") throw e;
};