filereader文件格式无法识别json和geojson

Question

I'm using filereader in Angular 2. I know that the file format can be accessed through:

 file.type

when I load for example txt, the type is: text/plain. But when I load a json or geojson the returned value is empty.

is there any reliable way to access to the types when its json, or geojson ?

Answer 1

The only reliable way to know if it is json or not would be to test the contents. Meaning run it through JSON.parse() . If it throws an exception out it isn't json (or is malformed json).

try {
  JSON.parse(fileContents);
} catch(e){
  console.log("Not a json file or file containing malformed json");
}

Mime types aren't stored as some piece of data inside files. It is usually determined by extension, magic number (binary header), etc. And depending on implementation, the browser is more than likely just going to look at the extension and use some lookup table. It would seem the browser you used did not have one set for a .json file

For instance loading a .json file in the below demo in Chrome will show application/json . Changing it to .txt will make it see it as text/plain . Changing it to some other extension will change it yet again.

 document.querySelector('input').addEventListener('change',function (){ document.body.insertAdjacentHTML('beforeend',this.files[0].type+"<br>"); }); 

 <input type="file" /><br>