demo D128
Formula entry: JavaScript Data Grid Demo
A spreadsheet formula typed into a cell, and the evaluator reach limit
formulaFunctions
This grid accepts a formula typed into a cell, like =quantity*unitPrice or =VAT(total), evaluates it against the row's own fields, and stores the number it produces as the cell's value. It lets the grid stand in for a spreadsheet where users expect to compute in a cell, with a defined limit on how far a formula reaches.
The configuration
<link rel="stylesheet" href="https://cdn.latticegrid.dev/lattice-grid@1.103.2/lattice-grid.min.css">
<script src="https://cdn.latticegrid.dev/lattice-grid@1.103.2/lattice-grid.min.js"></script>
<div id="grid" style="height: 540px"></div>
<script>
const grid = LatticeGrid.createGrid(document.getElementById('grid'), {
rowKey: 'id',
edit: true,
selection: 'multiple',
// Extra functions a formula may call. The number type's parse is handed
// them, so =VAT(...) resolves alongside the built-in library.
formulaFunctions: { VAT: (args) => Number(args[0]) * 1.2 },
columns: [
{ field: 'description', title: 'Description', layout: { flex: 1, min: 160, pin: 'start' } },
{ field: 'quantity', title: 'Qty', type: 'number', edit: true, layout: { width: 100 } },
{ field: 'unitPrice', title: 'Unit price', type: 'number', edit: true,
format: { style: 'currency', currency: 'GBP' }, layout: { width: 140 } },
{ field: 'discount', title: 'Discount', type: 'number', edit: true,
format: { style: 'percent', decimals: 0 }, layout: { width: 140 } },
{ id: 'lineNet', title: 'Line net', type: 'number',
format: { style: 'currency', currency: 'GBP' }, layout: { width: 130 },
value: {
deps: ['quantity', 'unitPrice', 'discount'],
compute: (d) => d.quantity * d.unitPrice * (1 - d.discount),
} },
{ field: 'total', title: 'Total', type: 'number',
format: { style: 'currency', currency: 'GBP' },
layout: { width: 140, pin: 'end' }, total: 'sum',
// The text editor on a number column, on purpose. The formula path lives
// in the number type's parse, and it only sees what the editor produces.
// The default number editor produces a number, so a formula never
// survives to reach the evaluator; the text editor hands text through
// untouched, so a column that accepts formulas asks for it here.
edit: { enabled: true, editor: 'text' } },
],
rows, // invoice lines: description, region, quantity, unitPrice, discount, total (empty)
});
// The number type's own parse is the function the commit path calls with
// whatever the editor produced, so calling it directly is the real evaluator
// on the real column. A formula is text a user typed, so its reach is the
// security boundary: there is no eval and no new Function behind it, only
// arithmetic, a closed list of functions and the row the cell stands in.
const column = grid.columns.all().find((c) => c.id === 'total');
const row = grid.rows.get(0);
const value = column.dataType.parse({
text: '=ROUND(quantity * unitPrice * (1 - discount), 2)',
value: null, data: row.data, row, column, grid,
});
</script>Typing a formula into a cell
Formula entry lets a cell hold =quantity*unitPrice or =VAT(total) instead of a value typed by hand. The grid parses the formula, evaluates it against the row’s own fields, and writes the number it produces into the cell. A developer reaches for it wherever a grid stands in for a spreadsheet a user already expects to type into: budget lines, pricing sheets, a total worked out from the other columns on the row rather than pasted in by hand. Lattice Grid registers the functions a formula may call through formulaFunctions, so a column can expose a small, named set such as VAT alongside arithmetic without the evaluator accepting an unbounded expression language. Entry works like inline editing: a formula is typed straight into the cell, committed with Enter, and evaluated once against that row, and its result is stored as a plain number. When you want a value that keeps tracking its inputs as they change, a computed column does exactly that: give the column the fields it depends on and it recalculates whenever any of them is written. Lattice Grid tracks a bounded evaluator reach, so a formula reaching across too many fields fails predictably at the configured limit rather than recursing until the tab stalls, which matters for a data grid rendering tens of thousands of rows where a runaway calculation would otherwise block the main thread. A screen reader announces the evaluated result, not the formula text, so a non-sighted user hears the number a sighted user sees, with the formula itself available on request when the cell is focused for editing.
How do I add a formula to a grid cell?
Set the column’s formulaFunctions to the functions a formula is allowed to call, mark the column editable, then type a formula into a cell starting with =, referring to the row’s fields by name. Lattice Grid parses it, evaluates it against that row, and stores the result as the cell’s value. For a value that recalculates as its inputs change, use a computed column instead: declare the fields it depends on and the grid re-runs it whenever one of them is written. An evaluator reach limit caps how far a single formula can walk, so a malformed or excessive reference fails at that boundary rather than degrading performance across the grid.