JavaScript FileReader中的readAsDataURL（）和readAsArrayBuffer（）以及readAsText（）之间的区别

Question

I have found a code of image uploading preview, I'm confused what's going on in FileRead() ?

var openFile = function(event) {
    var input = event.target;

    var reader = new FileReader();
    reader.onload = function() {
        var dataURL = reader.result;
        var output = document.getElementById('output');
        console.log(dataURL)
        output.src = dataURL;
    };
    reader.readAsDataURL(input.files[0]);
};

Answer 1

• .readAsDataURL() return a URL representing the file's data as a base64 encoded string

• .readAsArrayBuffer() return an ArrayBuffer representing the file's data

• .readAsText() return the file's data as a text string.

For more info check this FileReader doc.

Answer 2

readAsDataURL() will return a string that can be pasted into the url attribute of an HTML tag (eg: src= in an img). For an img tag, that will effectively display the image just like if src was an address to the file that was read. It is a transformation (bigger) of the original file content.

readAsText() will return a string of characters that can be parsed by JavaScript functions or displayed in textarea and likely to be understood by the user. This is usually useful for reading text files.

Answer 3

I think if you want to have a file upload functionality and then show the user a preview of the file they they chose from their PC and about to upload, then use .readAsDataURL() .

If you want to manipulate a text file, use .readAsText()

If you want to manipulate something like an image (convert a jpeg to PNG for example) then use .readAsArrayBuffer() .

There is a fourth method, .readAsBinaryString , but the Mozilla documentation suggests using .readAsArrayBuffer() instead.