How to Set a Custom File Name for Blob Downloads in JavaScript?

Setting Custom Blob File Name in JavaScript

When working with blobs in JavaScript, you may encounter a situation where you need to specify a custom file name instead of the default randomly generated one. This is particularly useful when you want to provide a meaningful file name for downloads or data exports.

Problem: Assigning Custom Blob File Name

The following code snippet creates a blob and attempts to download it through window.location, but the downloaded file has a random name like "bfefe410-8d9c-4883-86c5-d76c50a24a1d":

function newFile(data) {
    var json = JSON.stringify(data);
    var blob = new Blob([json], {type: "octet/stream"});
    var url = window.URL.createObjectURL(blob);
    window.location.assign(url);
}

Solution: Using the A Tag Trick

The only known method to assign a custom file name to a blob in JavaScript involves a trick using a hidden tag:

1. Create a hidden tag.
2. Set its href attribute to the blob's URL.
3. Set its download attribute to the desired file name.
4. Click on the tag programmatically.

var saveData = (function () {
    var a = document.createElement("a");
    document.body.appendChild(a);
    a.style = "display: none";
    return function (data, fileName) {
        var json = JSON.stringify(data),
            blob = new Blob([json], {type: "octet/stream"}),
            url = window.URL.createObjectURL(blob);
        a.href = url;
        a.download = fileName;
        a.click();
        window.URL.revokeObjectURL(url);
    };
}());

var data = { x: 42, s: "hello, world", d: new Date() },
    fileName = "my-download.json";

saveData(data, fileName);