README.md
In lt: Lightweight Tables via JSON Specs and JavaScript

LT

Lightweight tables for R, inspired by gt.

lt provides a small grammar of tables that covers the structure most reports need — titles, column spanners, row groups, footnotes, and number formatting — without the heavy dependency stack. It targets HTML only (no LaTeX or RTF), which keeps the implementation minimal: the entire runtime is a single vanilla-JS file (about 10 KB minified).

# CRAN version
install.packages("lt")

# development version
install.packages("lt", repos = "https://yihui.r-universe.dev")

You may also play with the package at https://pkg.yihui.org/lt/playground/ without installing it.

lt() creates a table object from a data frame. The lt_*() functions build on it via the pipe. See https://pkg.yihui.org/lt/examples/01-lt#sec:cheatsheet for a "cheat table" as an overview of these functions.

Structure

lt_header() — title and optional subtitle above the table.
lt_spanner() — column-spanner label spanning a group of columns; or auto-infer spanners from column name prefixes.
lt_group() — partition rows into labeled groups (rowspan or full-width separator style), either by column values or manual row indices.

Content & labels

lt_label() — override column header labels.
lt_footnote() — attach a numbered footnote to any region (title, subtitle, column, spanner, group, or body cells).
lt_note() — append a plain unnumbered note below the table.
lt_html() — render selected columns' cells as raw HTML instead of escaping them. For raw HTML in text (title, labels, footnotes, ...), wrap the text in I() in the corresponding function.

Formatting

lt_format() — numeric formatting: decimal places, thousand separators, prefix/suffix, percentage, scientific notation, etc.
lt_date() — date/datetime formatting using the browser locale.
lt_sub() — substitute specific values (e.g., replace 0 with "—" or NA with "n/a").
lt_merge() — merge several columns into one using a sprintf-style pattern.
lt_indent() — indent selected rows (useful for hierarchical row labels).

Appearance

lt_align() — set column alignment (left / center / right).
lt_width() — set column widths.
lt_style() — apply CSS classes or inline styles to cells, conditionally or unconditionally.
lt_css() — attach an external CSS file or URL to the table.

Column order

lt_move() — reorder columns.

Export

lt_export() — save a table to a file: .html (optionally baking a static <table> that needs no JavaScript to view), .pdf, or .png (rendered via a headless Chromium browser).

Shiny

lt_output() / render_lt() — Shiny UI and server bindings.

The R code below builds a table spec. Under the hood lt serializes it to a compact JSON object and ships it to the browser, where a tiny vanilla-JS runtime renders the <table>.

library(lt)

d = data.frame(
  Group = c("Treatment", "Treatment", "Control", "Control"),
  Endpoint = c("Primary", "Secondary", "Primary", "Secondary"),
  Estimate = c(0.6123, 0.7891, 0.4567, 0.5432),
  CI_Lower = c(0.4012, 0.5678, 0.2345, 0.3210),
  CI_Upper = c(0.8234, 1.0104, 0.6789, 0.7654),
  P_Value = c(0.0012, NA, 0.1234, NA)
)
lt(d) |>
  lt_group(~ Group) |>
  lt_header("Study Results", "Primary and secondary endpoints") |>
  lt_spanner(`95% CI` ~ CI_Lower + CI_Upper) |>
  lt_format(~ Estimate + CI_Lower + CI_Upper, decimals = 3) |>
  lt_sub(~ P_Value, missing = "—") |>
  lt_footnote("Two-sided p-value from log-rank test.", "column", ~ P_Value)

The same table can be built directly in JavaScript. Load lt.js once on the page, then call LT.build() from an inline <script> with the JSON spec:

<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/@xiee/utils/css/lt.min.css">
<script src="https://cdn.jsdelivr.net/npm/@xiee/utils/js/lt.min.js"></script>

<script>
LT.build({
  "data": {
    "Group":    ["Treatment", "Treatment", "Control", "Control"],
    "Endpoint": ["Primary", "Secondary", "Primary", "Secondary"],
    "Estimate": [0.6123, 0.7891, 0.4567, 0.5432],
    "CI_Lower": [0.4012, 0.5678, 0.2345, 0.3210],
    "CI_Upper": [0.8234, 1.0104, 0.6789, 0.7654],
    "P_Value":  [0.0012, null, 0.1234, null]
  },
  "ops": [
    { "type": "fmt_number", "columns": ["Estimate", "CI_Lower", "CI_Upper"], "decimals": 3 },
    { "type": "sub", "columns": ["P_Value"], "missing": "—" }
  ],
  "row_group": ["Group"],
  "header": { "title": "Study Results", "subtitle": "Primary and secondary endpoints" },
  "spanners": [{ "label": "95% CI", "columns": ["CI_Lower", "CI_Upper"] }],
  "footnotes": [{
    "text": "Two-sided p-value from log-rank test.",
    "location": { "type": "column_labels", "columns": ["P_Value"] }
  }]
});
</script>

LT.build() renders the table in place of the calling <script> tag. One lt.js inclusion handles any number of tables on the page.

You can find more examples at https://pkg.yihui.org/lt/examples.html.

This package was written with the help of Claude Code. lt is directly inspired by gt by Rich Iannone and the RStudio/Posit team. The grammar of tables that gt pioneered — layering titles, spanners, footnotes, and formatters onto a data frame — is a great idea; lt aims to provide a minimal re-implementation for contexts where a lighter footprint is preferred.