sloteffect

Slot-machine-style rolls for numbers, letters, and text in React. Each digit or character is its own reel that rolls up or down to its new value, wrapping through the charset, with a springy settle.

• Dependency-free — React is the only peer dependency (Web Animations API + a precomputed CSS linear() spring; no animation library, no JS spring loop).
• Three components — SlotNumber, SlotLetter, SlotText.
• Directional — direction="both" (random per reel, default), "up", or "down".
• Lean at rest — a character is a single glyph until it changes; the rolling strip is built only for the duration of a transition, then removed. Unchanged cells never animate, so a hero number costs a handful of DOM nodes, not hundreds.
• Any text, any direction — SlotText segments by grapheme (emoji, combining marks, surrogate pairs stay intact), keeps natural spacing/kerning, and orders LTR or RTL automatically.
• Accessible — the value is the aria-label; reels are presentational; prefers-reduced-motion snaps instantly.
• Tiny — ~5 KB ESM, tree-shakeable, ships ESM + CJS + types.

▶ Live demo

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Install

# https://www.npmjs.com/package/sloteffect

bun add sloteffect
npm i sloteffect
pnpm add sloteffect
yarn add sloteffect

React 18 or 19 is a peer dependency.

Quick start

import { SlotNumber, SlotLetter, SlotText } from "sloteffect";

// Currency / percent / compact — anything Intl.NumberFormat can format
<SlotNumber value={total} format={{ style: "currency", currency: "USD", maximumFractionDigits: 0 }} />

// A single rolling character
<SlotLetter char={grade} />

// An arbitrary string: letters and digits roll, the rest stays put
<SlotText text="JACKPOT 7" />

Place a component inside a styled element — it inherits font-family, font-size, and color from the parent.

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Components

SlotNumber

Formats a value with Intl.NumberFormat (plus an optional suffix) and rolls the digits. Separators, currency symbols, and the suffix stay static. Accepts any number (decimals, negatives, NaN/Infinity), a bigint, or an already-formatted string (passed through verbatim).

<SlotNumber value={0.813} format={{ style: "percent", minimumFractionDigits: 1 }} />   // 81.3%
<SlotNumber value={1234.5} format={{ style: "currency", currency: "USD" }} cents counter />  // $1,234.⁵⁰
<SlotNumber value={9007199254740993n} />                                               // bigint

Prop	Type	Default	Description
value	number | bigint | string	—	Number/bigint to format, or a preformatted string.
format	Intl.NumberFormatOptions	—	Style, currency, fraction digits, etc.
locales	string | string[]	"en-US"	BCP 47 locale(s).
suffix	string	""	Static text appended after the number (e.g. "/yr").
direction	"both" | "up" | "down"	"both"	Roll direction.
randomSpin	boolean	false	Stagger each digit's spin/settle randomly per play.
cents	boolean	false	Render the two fractional digits at 90% size, bottom-aligned (the decimal point stays full size). Forces two fraction digits.
counter	boolean	false	Roll the digits like a gear-reduction odometer: each rolls from the previous value by however many of its own place-steps the value crossed, so low digits blur while the ten-thousands and up barely move. Best for a value that changes continuously (e.g. a slider); pass a stable format reference.
className / style	—	—	Passed to the container.

cents and counter read the value's digit places via Intl.NumberFormat.formatToParts; they're skipped when value is a preformatted string.

SlotLetter

A single character rolling through its case's alphabet (A–Z or a–z), wrapping around. Non-letters render static.

<SlotLetter char="Q" direction="up" />

Prop	Type	Default	Description
char	string	—	The character to display.
direction	"both" | "up" | "down"	"both"	Roll direction.
className / style	—	—	Passed to the container.

SlotText

Animates any string in any script. The text is split into grapheme clusters, and each cluster is a slot cell: digits and same-case Latin letters roll through their charset, while every other glyph (other scripts, emoji, punctuation) flips cleanly to its new value. At rest each cell is normal text, so spacing, kerning, and LTR/RTL ordering are correct; cells are right-anchored so trailing positions keep rolling as the string grows or shrinks.

<SlotText text="Level 12" direction="down" />
<SlotText text="東京タワー" />          // CJK
<SlotText text="שלום עולם" />          // right-to-left, auto-detected

Prop	Type	Default	Description
text	string | number | bigint	—	The string to display.
direction	"both" | "up" | "down"	"both"	Roll direction.
dir	"auto" | "ltr" | "rtl"	"auto"	Writing direction; auto infers it from the content.
className / style	—	—	Passed to the container.

Note on connected scripts. Because each grapheme animates in its own cell, scripts with contextual joining (e.g. Arabic) render in isolated forms while rolling. Latin, CJK, Hebrew, emoji, and digits are unaffected.

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Direction

Every component accepts direction:

Value	Behavior
"both" (default)	Each reel flips a coin — up or down — independently.
"up"	All reels roll upward (charset index increases), wrapping.
"down"	All reels roll downward, wrapping.

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Integration notes

• Use tabular-nums. Set font-variant-numeric: tabular-nums on the parent so digits keep an identical width and the layout doesn't shiver.
• Line-height lives outside. Reel cells are exactly 1em tall; give the parent line-height ≥ 1.15 (serif faces especially) so nothing clips.
• Reserve it for hero numbers. Animate key stats and answer-numbers — not tables, axis ticks, or tooltips that re-render wholesale.
• Reduced motion. Under prefers-reduced-motion: reduce the reels snap to the final value with no animation. (Headless/preview browsers often report reduced motion — stub matchMedia to observe the roll in tests.)
• Ordered vs. flip. Latin 0–9 / A–Z / a–z roll through their charset (the authentic reel). Any other glyph pair (other scripts, emoji, cross-charset) flips directly from old to new. For non-Latin numerals that should roll like digits, format with the nu-latn numbering system.

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Development

bun install
bun run typecheck   # tsc --noEmit (strict)
bun run lint        # biome check
bun run test        # vitest (direction/wraparound logic)
bun run build       # tsup → dist/ (ESM + CJS + .d.ts)

The showcase (index.html) is a single, build-free file deployed to GitHub Pages by .github/workflows/pages.yml.

License

MIT © Jauder Ho