Registration behavior and sample item distinctions
This section details initial behavior, item registration mechanics, and input validation rules for the subscription organizer and monthly expense conversion sheet.
When opening this tool for the first time, if no data exists in browser localStorage under the subs_data key or if existing records cannot be parsed due to corruption, the tool presents four built-in sample subscriptions: Video Streaming (Netflix, etc.) at 1,490 yen, Music Streaming (Spotify/Apple Music, etc.) at 1,080 yen, Cloud Storage (iCloud/Drive, etc.) at 400 yen, and News/Business Magazine Subscription at 9,800 yen annually.
If a loading error occurs, the system is designed not to overwrite or destroy previously stored records automatically. When adding a new service, strict input validation is enforced.
Service names must not exceed 200 characters, prices must be finite numbers greater than 0 and up to 1,000,000,000 yen, and total registered items are capped at 500.
Internally, a unique identifier is generated via crypto.randomUUID() for each entry, distinguishing services even if they share identical titles.
If invalid inputs such as negative amounts or excessive lengths are submitted, a warning stating Service name must be 200 characters or fewer, price must be greater than 0 and up to 1 billion yen, and total entries must not exceed 500. is displayed, and addition to the list is aborted.
Check input formats prior to submission.
Three-cycle conversion formulas and rounding rules
This tool allows users to select from three payment billing cycles—weekly, monthly, or annual—for each registered subscription and normalizes them into unified monthly equivalent figures.
Specific conversion formulas reconcile services across differing cycles. Monthly subscriptions use the entered amount directly. Weekly subscriptions multiply the entered amount by 4.33 to derive a monthly value; for example, 500 yen weekly becomes 500 × 4.33 = 2,165 yen.
Annual subscriptions divide the entered amount by 12; for example, 12,000 yen annually converts to 1,000 yen monthly. After summing monthly equivalents across all services, rounding is applied when computing total monthly expenditure.
This rounded total monthly spend forms the baseline for other timeframe metrics: total annual expenditure is calculated as total monthly spend multiplied by 12, while daily equivalent cost divides total monthly spend by 30 and rounds the result.
Values are formatted on screen in Japanese yen notation without decimal places. Separating individual conversion rounding from final total aggregation suppresses display drift while visualizing clear integer costs.
Category breakdowns and subscription review insights
Each registered subscription is categorized into one of five fixed categories: Entertainment, Tools / Work, Personal Development, Living / Fitness, and Other.
These categories provide insights to identify spending concentrations and support subscription reviews. Category breakdowns sum the monthly equivalents of all services matching the selected category.
Next, the tool computes the proportion of that category's total relative to overall total monthly expenditure. This proportion is formatted to one decimal place and displayed as a progress bar for each category.
This visualization helps verify whether spending is heavily skewed toward entertainment or dominated by software tools.
Scope of browser local storage
Subscription data entered into this tool is processed and saved exclusively within the browser built-in localStorage. Whenever an addition or deletion occurs, the entire current list is serialized into a JSON string and saved under the subs_data key in localStorage.
This allows previous states to be restored by retrieving and deserializing the string on subsequent visits. However, storage functionality depends on browser settings.
If private browsing mode is active or storage limits are exceeded, saving may fail. When errors occur during write operations, the tool catches the exception and displays a warning stating `Could not save data.
Please check price, entry count, and storage settings. The current list was not modified.` Under this error handling, the list currently displayed remains untouched and existing stored data is not overwritten.
Operating within local browser storage reflects the data boundary of this tool and does not dictate broader network activity for the overall site.
JSON backup export and replacement import
Because automated synchronization across different devices or browsers is not supported, the tool provides JSON backup exports and file replacement imports as alternatives.
Using the export feature downloads the current subscription list as a JSON file named subscriptions_backup.json. The list is converted into a URI-encoded string and downloaded locally.
The import feature reads a locally stored file as plain text. The imported content is parsed as JSON and subjected to the aforementioned validations (character lengths, numeric bounds, 500-item ceiling, and the 5 valid categories).
If the file contains invalid formats or out-of-range values, a warning is raised and the current list remains unchanged. If the data passes validation, a confirmation dialog appears: `This will replace the current list with the imported JSON.
Please back up first if necessary.` Only upon confirmation is the current list and saved storage overwritten with the imported content.
This function replaces data entirely rather than merging diffs with existing entries.
Differences from actual contracts and real-world billing
This tool is a static simulator calculating costs based on manually entered numbers and does not directly manage, execute, or renew actual subscription agreements.
It does not track actual billing dates or payment cycles, nor does it send push notifications ahead of automated contract renewals.
Furthermore, it lacks integrations with bank accounts, credit cards, or external budgeting applications to sync transaction data automatically.
The system simply displays monthly and annual figures calculated using static multipliers from chosen cycles (weekly, monthly, annual).
In addition, it does not track dynamic pricing changes, such as provider fee revisions, introductory free trials, promotional discounts, or foreign currency exchange fluctuations.
If an actual billing charge changes, users must manually delete and re-enter the updated service. Nor does it compute exact daily prorated costs accounting for variable month lengths (such as 31 or 28 days), or provide encrypted cloud sync across devices.
The tool is strictly designed to apply simplified rounding logic to finite user-entered data and maintain lists in the browser.