________________________________________
Comprehensive and Detailed Explanation From Exact Extract JavaScript Knowledge The File API in browsers provides the FileReader object to read file contents selected from <input type="file">.
Important knowledge points:
new FileReader() creates a file-reading object.
.readAsDataURL(file) reads a file and produces a Base64 URL string.
The "load" event fires when the file has finished reading.
reader.result contains the data URL after reading completes.
Therefore, the correct implementation must:
Create a FileReader instance:
const reader = new FileReader();
Call:
reader.readAsDataURL(file);
Use the load event handler to assign the image preview:
preview.src = reader.result;
Option B is the only option that matches valid JavaScript File API usage.
Option A is incorrect because File is not a constructor for reading files.
Option C is incorrect because URL.createObjectURL(file) must be assigned directly as a URL, not used with reader.result.
________________________________________
JavaScript Knowledge Reference (text-only)
The file-reading interface in browsers is FileReader.
readAsDataURL() loads files as Base64 data URLs.
The load event indicates when the reader has finished and reader.result is available.