Create and save a file with JavaScript

In-Depth Walkthrough: What you need to know

Crafting Blobs: MIME types matter

Let's not just create blobs without giving it some thought. Encode your Blob data to match a suitable MIME type. Here's how you do it for JSON:

const lifefulJSON = JSON.stringify({ key: 'value' });
const jsonBlob = new Blob([lifefulJSON], {type: 'application/json'});
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And here's how you would do it for CSVs:

const csvData = 'Header1,Header2\nvalue1,value2'; // Millenials will say this is an old-school Excel
const csvBlob = new Blob([csvData], {type: 'text/csv'}); // Creates blob, but not a horrifying creature
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Cross-Browser Compatibility: IE, I'm looking at you!

Oh, IE, the resident problem child on the web. IE requires some coddling using msSaveOrOpenBlob. This is how you make everyone play nice:

if (window.navigator.msSaveOrOpenBlob) {
  window.navigator.msSaveOrOpenBlob(blob, 'filename.txt'); // IE's special treatment
} else {
  // Regular awesome code here
}
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Non-compliant browsers & Security constraints

Safari, the rebel without a cause, requires a different approach. Let's wrangle it with window.open() to prompt manual download:

if (/constructor/i.test(window.HTMLElement) || (function (p) { return p.toString() === "[object SafariRemoteNotification]"; })(!window['safari'] || (typeof safari !== 'undefined' && safari.pushNotification))) {
  // Safari says, "What's a Blob? Let's just open it."
  window.open(link.href);
}
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Give a thought for the security constraints: JavaScript cannot choose the file save location due to security limits set by browsers.

Actionable Implementation: Button onClick event

Let's make this process trigger with a button onClick event:

HTML:

<button id="saveBtn">Save File</button> <!-- Basic download button -->
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JavaScript:

document.getElementById('saveBtn').addEventListener('click', function() {
  // File creation code here
});
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Edge cases and other considerations

Dynamic filenames and content

If dynamic filenames or content is your poison, let's manipulate them before the Blob is created:

const userName = 'JohnDoe';
const dynamicContent = `Welcome, ${userName}!`; // Dynamic content
const dynamicFilename = `welcome_${userName}.txt`; // Dynamic file name

const dynamicBlob = new Blob([dynamicContent], {type: 'text/plain'}); // Dynamic blob creation
const dynamicLink = document.createElement('a'); // Dynamic link creation!
dynamicLink.href = URL.createObjectURL(dynamicBlob);
dynamicLink.download = dynamicFilename;
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Error Handling and User feedback

Handling errors may seem like an "Errorception", but it's necessary. Also, users love feedback. Feed them:

try {
  link.click(); // Attempt to click link
} catch (err) { // Oops, caught something!
  console.error('Error during file download', err); // Log it
  // Give the user a pat on the back, or an error message. They will appreciate it.
}
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Handling Larger Files

If your files or data are larger than your average cat video, consider breaking them down into chunks or streams. This prevents high CPU usage and potential browser crashes.