Charlie

Check a 17-character vehicle identification number and its check digit; enforced for North American VINs.

Check a UK VAT registration number against HMRC's mod-97 checksum, including the post-2010 9755 variant.

Check an HMRC Unique Taxpayer Reference's modulus 11 check digit, accepting the K and spacing people type.

Check an HMRC Unique Taxpayer Reference's modulus 11 check digit, accepting the K and spacing people type.

Check an HMRC Unique Taxpayer Reference's modulus 11 check digit, accepting the K and spacing people type.

Check a UK sort code and account number with the Vocalink/Pay.UK modulus rules, against a table you supply.

Check a UK National Insurance number's format and HMRC's disallowed prefix letters, and normalise it.

Check a UK National Insurance number's format and HMRC's disallowed prefix letters, and normalise it.

Parse Vocalink's VALACDOS.txt and SCSUBTAB.txt into the table UK sort code modulus checking needs.

Check a Companies House company number's format and prefix (SC, NI, OC, LP...) and zero-pad it to eight characters.

Check a Companies House company number's format and prefix (SC, NI, OC, LP...) and zero-pad it to eight characters.

Check a Companies House company number's format and prefix (SC, NI, OC, LP...) and zero-pad it to eight characters.

Normalise a phone number to E.164 (+447700900123) using a default country's trunk and dialling prefixes.

Normalise a phone number to E.164 (+447700900123) using a default country's trunk and dialling prefixes.

Check a digit string against the Luhn mod-10 checksum used by payment cards and many identifiers.

Check a 20-character Legal Entity Identifier against its ISO 17442 / ISO 7064 MOD 97-10 check digits.

Check an ISBN-10 or ISBN-13 and convert between them; the normalised form is always the ISBN-13.

Check an IBAN against the ISO 13616 mod-97-10 checksum and the registered length for its country.

Check an EAN-8, UPC-A, EAN-13 or GTIN-14 barcode number against the GS1 mod-10 check digit.

Check an EAN-8, UPC-A, EAN-13 or GTIN-14 barcode number against the GS1 mod-10 check digit.

Check the format of a SWIFT/BIC code (8 or 11 characters) and split it into its parts.

Parse "2.5 kg", "1,200 m" or "12 ft 6 in" into an exact decimal value and a canonical unit symbol, strictly.

How many todos carry each tag, for a tag sidebar: most used first, open todos only unless asked.

Counts for a todo list's header or footer: total, active, done, overdue, due today, percent done and "3 items left".

Sort todos by manual order, due date, priority, creation time or title, stably, with sensible tie-breakers.

When a repeating todo's due date is moved by hand, move its series with it: the new date becomes the anchor.

English text for a repeating todo's chip: "Every weekday", "Every 2 weeks on Monday", "Every month on the 31st".

Read a tags box as typed, "#Home, work errands", into normalised tags: split on spaces and commas.

Read a Repeat box, "every 2 weeks" or "every monday on 2026-10-05", as a todo's recurrence, or null.

Turn one typed line like "Pay rent tomorrow !high #home every month" into a todo draft.

Read a typed due date like "tomorrow", "next friday", "in 2 weeks" or "2026-10-01" as a date, relative to today.

Merge imported todos into a list by id: an imported todo replaces the one with its id, new ids are appended.

The to-do list reducer: add, edit, complete, remove, clear and reorder todos, each returning a new list.

The to-do list reducer: add, edit, complete, remove, clear and reorder todos, each returning a new list.

The next due date of a repeating todo once it is done: daily, weekdays, weekly, monthly or yearly, month-end safe.

Read todos from a CSV file (RFC 4180) as todo.export-csv writes it, checking every row, with row-numbered errors.

Read todos from a CSV file (RFC 4180) as todo.export-csv writes it, checking every row, with row-numbered errors.

Split todos into Overdue, Today, Tomorrow, This week, Later, No date and Done sections, in that order.

Split todos into Overdue, Today, Tomorrow, This week, Later, No date and Done sections, in that order.

The todos a list view shows: all, active or completed, optionally one tag, and a search box, in input order.

Clean up tags as typed: trim, drop the #, lower-case slug form, drop empties and duplicates, keep the order.

Write todos as a CSV file (RFC 4180) with one column per field, for backups, spreadsheets and other apps.

The short English label shown next to a todo: "2 days overdue", "Due today", "Due tomorrow", "Due Fri 2 Oct".

Where a todo stands against its due date today: done, overdue, today, tomorrow, this week, later or no date.

The to-do item type (Todo, TodoDraft, Recurrence) and its validators, with a message per field.

Parse durations written as 1h30m, 90m or 01:30, add them up in whole minutes, and format the total.

Seconds left until a Unix time, whether it has passed, and a timer text like 4:05, for tokens or retry waits.

Shorten text to a maximum length on a word boundary and add an ellipsis, counting Unicode code points.

Turn a title into a lowercase ASCII URL slug, transliterating common accented Latin letters.

Mask all but the last n characters of a card, account or phone number, optionally keeping separators.

Initials from a personal name for avatars and signatures: "Jean-Paul Sartre" is "JPS".

Roaming charges for a trip abroad from a dated tariff table of zones: daily fees, data, calls and texts.

Roaming charges for a trip abroad from a dated tariff table of zones: daily fees, data, calls and texts.

Mobile data allowance used and remaining, and the overage charge in whole blocks with an optional spend cap.

Rate a call from a rate deck: longest-prefix destination match, billing increments, connection fee and minimum charge.

Rate a call from a rate deck: longest-prefix destination match, billing increments, connection fee and minimum charge.

Price metered usage against tiers, graduated or volume, with an optional flat fee per tier.

When a free trial ends and the first payment is due, from the trial's start date and length in days.

Mid-cycle plan change: credit for unused time on the old plan and charge for the rest of the period on the new one.

Monthly and annual recurring revenue from a list of subscriptions, normalising intervals and leaving out trials.

Monthly and annual recurring revenue from a list of subscriptions, normalising intervals and leaving out trials.

The next renewal date after a given day, from a billing anchor and interval; month-end safe.

Customer lifetime value, LTV to CAC ratio and CAC payback months from ARPA, gross margin, churn and CAC.

A subscription renewal invoice: plan, add-ons and usage, coupon discounts by billing period, and VAT.

Retry dates for a failed subscription payment from a retry policy, in calendar or business days.

Logo churn, gross and net revenue churn, and gross and net revenue retention for one period, in basis points.

Weighted average of integers, computed exactly and rounded once to stated decimals with an explicit rounding mode.

Population or sample standard deviation of a list of numbers, two-pass, rounded to stated decimals.

Population or sample standard deviation of a list of numbers, two-pass, rounded to stated decimals.

Simple or exponential moving average over a series, one value per full window, rounded to stated decimals.

Simple or exponential moving average over a series, one value per full window, rounded to stated decimals.

Mean, median and mode of a list of integers, with the mean and median as exact fractions.

Build a deterministic stock-keeping unit code from a prefix and product attributes.

Delivery charge from a dated rate table by service, zone, chargeable weight and size, with a free-over threshold.

Delivery charge from a dated rate table by service, zone, chargeable weight and size, with a free-over threshold.

Chargeable parcel weight: the greater of actual and volumetric (dimensional) weight, rounded up to a billing step.

US sales tax on a sale: the state's base rate from dated data plus a local rate the caller supplies.

US sales tax on a sale: the state's base rate from dated data plus a local rate the caller supplies.

Refund for returned items, sharing basket discounts back across the sale so every refund is exact to the penny.

Snap a price to a price point: charm pricing (.99, .95), nearest 5p or 10p, rounding up, down or to nearest.

Unit price for shelf-edge labels: price per kg, per 100 g, per litre, per 100 ml, per metre or per item.

Unit price for shelf-edge labels: price per kg, per 100 g, per litre, per 100 ml, per metre or per item.

EU distance-selling VAT: which member state's standard rate applies under OSS and the EUR 10,000 threshold, and the VAT.

EU distance-selling VAT: which member state's standard rate applies under OSS and the EUR 10,000 threshold, and the VAT.

Loyalty points for a purchase: redeem in blocks, earn on what is paid, with tier multipliers.

Apply a payment from a gift card, redeeming part of the balance or all of it, and say what is still due.

Check a coupon against a basket: dates, usage limits, eligible products and minimum spend, with reason codes.

Check a coupon against a basket: dates, usage limits, eligible products and minimum spend, with reason codes.

Total a basket: line prices, promotions, a basket discount, VAT per line and delivery, into one checked total.

Apply BOGOF, 3-for-2, percent off, amount off and multibuy deals to a basket, best for the customer, allocated to lines.

A labelled multi-line text box with hint, error and an optional live character or word count (GOV.UK).

A labelled multi-line text box with hint, error and an optional live character or word count (GOV.UK).

A labelled single-line text input with hint, error message, autocomplete, prefix and suffix (GOV.UK text input).

A labelled single-line text input with hint, error message, autocomplete, prefix and suffix (GOV.UK text input).

An optional stylesheet for the react.form elements' caps-* classes, themed by --caps-form-* custom properties.

An optional stylesheet for the react.form elements' caps-* classes, themed by --caps-form-* custom properties.

A labelled drop-down list with a hint, error message and an optional empty first option (GOV.UK select).

A labelled drop-down list with a hint, error message and an optional empty first option (GOV.UK select).

Choose exactly one of several radios, with item hints, an "or" divider and inline yes/no (GOV.UK radios).

Choose exactly one of several radios, with item hints, an "or" divider and inline yes/no (GOV.UK radios).