How to create a modified copy of a File object in

问题:

Properties of files received from an <input type="file"> are read-only.

For example, the following attempt to re-write file.name would either fail silently or throw TypeError: Cannot assign to read only property 'name' of object '#<File>'.

<input onchange="onchange" type="file">

onchange = (event) => {
    const file = event.target.files[0];
    file.name = 'foo';
}

Attempting to create a copy via Object.assign({}, file) fails (creates an empty object).

So how does one clone a File object?

回答1:

My solution lay in the File constructor:

https://developer.mozilla.org/en-US/docs/Web/API/File#Implementation_notes

Which itself is an extension of Blob:

https://developer.mozilla.org/en-US/docs/Web/API/Blob/Blob

let file = event.target.files[0];
if (this.props.distro) {
    const name = 'new-name-here' + // Concat with file extension.
        file.name.substring(file.name.lastIndexOf('.'));
    // Instantiate copy of file, giving it new name.
    file = new File([file], name, { type: file.type });
}

Note the first argument to File() must be an array, not simply the original file.

回答2:

You can use FormData.prototype.append(), which also converts a Blob to a File object.

let file = event.target.files[0];
let data = new FormData();
data.append("file", file, file.name);
let _file = data.get("file");