> ## Documentation Index > Fetch the complete documentation index at: https://learn.nextedy.com/llms.txt > Use this file to discover all available pages before exploring further. # Add a Computed Column > Derive a column's content from an expression in Nextedy POWERSHEET — either stored back to an entity property, or displayed only, with no property behind it. export const LastReviewed = ({date}) => { if (!date) return null; const formatted = new Date(`${date}T00:00:00Z`).toLocaleDateString("en-US", { year: "numeric", month: "long", day: "numeric", timeZone: "UTC" }); return

Last reviewed on {formatted}

; }; A **computed column** derives its content from a `value` expression instead of reading a stored property directly. Powersheet computes it in the browser every time the sheet renders. There are two kinds, and one character in the binding decides which you get: * a **stored** computed column writes its result back to the property it binds to, and * a **display-only** computed column stores nothing — it exists in the sheet and nowhere else. ## Prerequisites * A working sheet configuration YAML file * A data model whose properties you want to compute from * Familiarity with [dynamic value expressions](/powersheet/reference/sheet-config/dynamic-expressions) (`() =>` syntax) ## Decide Whether the Value Should Be Stored The binding key decides. A key prefixed with `$` is **unbound**: it names a column, not a property. | Binding | `value` | Result | | -------- | ------- | ------------------------------------------------------------------------------------------------------------------------------- | | `total` | not set | Ordinary stored column — the user edits it | | `total` | set | Computed **and stored** — the result is written to `total`, marks the row modified, and persists on save. Read-only to the user | | `$total` | set | Computed and **displayed only** — nothing is stored, nothing is saved. Read-only to the user | | `$total` | not set | Configuration error — an unbound column has nothing to derive its content from | **`value` is about data; `render` and `display` are about appearance.** Do not reach for `render` to avoid writing data — `render` styles whatever the cell already holds. If the value should never be stored, make the column unbound with `$`. If it should be stored, bind it to a real property. Bind the column to a real property and give it a `value` expression. The result is written to that property, so it saves with the rest of the row: ```yaml theme={null} columns: total: title: Total value: "() => context.entity.count * context.entity.rate" format: "c0$" ``` The user cannot type into the column — a computed column is read-only, because its content comes from the expression rather than from input. Editing `count` or `rate` recomputes it, marks the row modified, and the new total persists on the next save. Prefix the binding key with `$` to name a column that has no property behind it: ```yaml theme={null} columns: $riskScore: title: Risk Score value: "() => context.entity.Probability * context.entity.Severity" valueType: number ``` Nothing is written anywhere. The column appears in the sheet, is read-only, and leaves the work item untouched — saving the row does not create or update a `riskScore` field. The `$` marker belongs on the **last** segment only. Everything before it is an ordinary path that must resolve in the data model, and it decides which entity the column reads from: ```yaml theme={null} columns: # derives from the root entity $riskScore: value: "() => context.entity.Probability * context.entity.Severity" # derives from the expanded system requirement systemRequirements.systemRequirement.$coverage: value: "() => context.entity.verifiedCount + ' / ' + context.entity.totalCount" ``` **`$total` and `total` can coexist.** The marker stays part of the column's internal identity, so a display-only `$total` does not collide with a real `total` property in the same sheet. Give them different `title` values so readers can tell them apart. If you leave `title` out, the header shows the name without the marker — `$riskScore` displays as `riskScore`. A display-only column has no metadata to take its type from, so declare it with `valueType`: | `valueType` | Use for | Notes | | ----------- | ---------------------------- | -------------------------------------------------------- | | `string` | Text, labels, concatenations | The default when `valueType` is omitted | | `number` | Scores, counts, sums, prices | Enables `format` and `aggregate` | | `date` | Dates and date-times | Formats as `MMM dd, yyyy` unless `format` says otherwise | ```yaml theme={null} columns: $riskScore: title: Risk Score value: "() => context.entity.Probability * context.entity.Severity" valueType: number format: "n0" $reviewDue: title: Review Due value: "() => context.entity.approvedOn" valueType: date format: "yyyy-MM-dd" ``` **`valueType` is for unbound columns only.** A bound column takes its type from the data model, so declaring `valueType` on one is rejected. And because there is no metadata to consult, `valueType: date` cannot distinguish a date from a date-time — set `format` if you need the time part shown or hidden. A numeric display-only column can be aggregated into group rows like any other numeric column: ```yaml theme={null} columns: $riskScore: title: Risk Score value: "() => context.entity.Probability * context.entity.Severity" valueType: number aggregate: sum groupBy: true ``` `aggregate` requires `valueType: number` — declaring it on a text or date column is rejected. Inside an unbound column's `value` expression you can reach past the current entity: | Expression | Gives you | | ---------------- | --------------------------------------------------------------------------- | | `context.entity` | The entity at the column's own level — the one the binding path resolves to | | `context.source` | The entity one level up, the one this level was navigated from | | `context.row` | The whole row, as `context.row.entities[]` across every level | ```yaml theme={null} columns: systemRequirements.systemRequirement.$origin: title: Origin value: "() => `${context.source.id} → ${context.entity.id}`" ``` **`context.value` is empty in a `value` expression.** A computed column has no stored cell value to start from, so `context.value` is `undefined`. Read the properties you need from `context.entity` instead. ## What a Display-Only Column Cannot Do A display-only column stores nothing and resolves no metadata, so several column properties do not apply to it. Each one is rejected outright rather than ignored: | Property | Why it is rejected | | --------------------------------------- | -------------------------------------------------------------------- | | `isRequired` | The column is read-only and holds no stored data | | `multiItem` | It binds no navigation property | | `list` | The stages that resolve enumerations and references never run for it | | `valueType` on a **bound** column | A bound column takes its type from the data model | | `aggregate` without `valueType: number` | Only numeric derived values aggregate meaningfully | `render`, `renderers`, `format`, `groupBy`, `sort`, `filter`, `columnGroup`, and the styling properties all work normally. ## Verify After saving the configuration, do a full browser reload or re-open the document — the in-sheet refresh button reloads data, not the configuration. You should now see: * The computed column shows its derived value on every row * Clicking a cell does not enter edit mode, and pasting into the column changes nothing * Editing a property the expression reads updates the derived cell immediately * For a **stored** computed column, the row is marked modified and the value persists after save * For a **display-only** column, saving leaves the work item untouched * Grouping, sorting, filtering, sheet search, and Excel export all use the derived value **Grouping and sorting keep the keys they were built with.** A derived cell repaints as soon as a property it reads changes, but an active grouping or sort is not rebuilt from the new values. Reload the sheet to regroup or resort on the updated keys. ## Troubleshooting | Message | Cause | | ------------------------------------------------------ | --------------------------------------------------------------------------------------------------------------------- | | `The '$' marker is only allowed on the last segment.` | The marker is on a path segment other than the leaf | | `The '$' marker needs a name (eg. '$riskScore').` | The binding is a bare `$` with nothing after it | | `The path before '$' does not resolve.` | The segments preceding the marker are not a valid path in the data model | | `has no 'value' expression to derive its content from` | A `$`-prefixed column without a `value` expression | | Column is blank on some rows | The row has no entity at that level. The expression still runs, but `context.entity` is `undefined` — guard with `?.` | ## See Also * [Add a Column](/powersheet/guides/sheet-configuration/add-column) -- basic column setup and property binding * [Configure Dynamic Expressions](/powersheet/guides/sheet-configuration/configure-dynamic-expressions) -- `() =>` expressions across the whole sheet configuration * [Configure Read-Only Column](/powersheet/guides/sheet-configuration/configure-read-only-column) -- how read-only is resolved * [Use JavaScript in Display and Render](/powersheet/guides/sheet-configuration/use-javascript-display) -- change appearance without changing data * [Create a Computed Property](/powersheet/guides/server-rendering/create-computed-property) -- compute on the server with Velocity instead * [Columns](/powersheet/reference/sheet-config/columns) -- complete column property reference * [Binding Syntax](/powersheet/reference/sheet-config/binding-syntax) -- binding path patterns, including the `$` marker * [Dynamic Value Expressions Reference](/powersheet/reference/sheet-config/dynamic-expressions) -- the context object and where each expression runs