未定义的数组

  public onChange(event: Event) {
    let files = event.target['files'];
    let list: string[];
    console.log(files);
    for (var i = 0; i < files.length; i++) {
      if (FileReader && files && files.length) {
        let fileReader = new FileReader();
        fileReader.onload = () => {
          let infoFile: string = fileReader.result;   
          list.push(infoFile);         
        };
        fileReader.readAsDataURL(files[i]);
      }
    }
    console.log(list);
    let preview = document.querySelector('img');
    preview.src = list[0];
  };

我在我的component.ts中有方法，当我点击input type =file时，名为medthod onChange ）但是当在控制台中留下循环时，我有消息该数组是未定义的。我检查fileReader.result不是空的为什么发生这种情况？如何解决这个问题的一些想法。

I have method in my component.ts when i click input type="file" called medthod onChange() but when left loop for in console i have message that array is undefined. I check that fileReader.result is not empty Why is this happening? How to solve this problem some idea.