[Canonical page]() [Full documentation index]() # Editing a project This guide covers how to change a live [`Project`]() as the user works: adding, updating and removing tasks, links, resources and assignments, rescheduling after the edit, and recovering when part of an edit is refused. ## How editing works A [`Project`]() holds one accepted state: the data from the last [`solve(data)`]() plus every edit since, with the dates the last solve accepted. You don't send the whole plan again for each change. You describe the change as a [`SchedulingChanges`]() object and pass it to [`project.apply()`](). Editing takes two separate calls: 1. `apply(changes)` writes the edit into the accepted state and marks the project unsolved. It doesn't schedule anything. 2. `solve(null)` schedules the edited state, accepts the new dates and returns what moved. Between the two calls the project has no baseline. `preview`, `loadProfile`, `evaluate`, `slots` and `suggestAssignments` throw a [`ProjectError`]() with code `"no-baseline"` until you solve. You can call `apply` several times before one `solve(null)`. The edits accumulate. Both calls are synchronous, like every `Project` method. `apply` accepts what it can. Each operation in the change set applies or is rejected on its own. A rejection doesn't throw. It's listed in the returned report, and it sets `needsResync`. From then on, `solve(null)`, `apply` and `level` throw `ProjectError` with code `"needs-resync"`. Your model and the project's state may now disagree, and the only way back is to send the full data again with `solve(data)`. When `apply` itself throws, nothing was applied. A `ProjectError` means the call didn't fit the project's state: `"needs-resync"` after an earlier rejection, or `"stale-baseline"` when the change set names a baseline that isn't current anymore. An [`InputError`]() means the change set is malformed, for example an unknown key such as `tasks.delete`. See [Error handling](). ## Change sets A [`SchedulingChanges`]() object groups the edits by record kind. Each group has three optional arrays: `add`, `update` and `remove`. Leave out any group or array you don't need. | Group | `add` | `update` | `remove` | |---|---|---|---| | `tasks` | [`TaskData`]() | [`TaskUpdate`]() | task ids | | `links` | [`LinkAdd`]() | [`LinkUpdate`]() | link ids | | `resources` | [`ResourceData`]() | [`ResourceUpdate`]() | resource ids | | `assignments` | [`AssignmentData`]() | [`AssignmentUpdate`]() | assignment ids | Additions use the same records as [`ProjectData`](), except links: a `LinkAdd` is a [`LinkData`]() whose `id` is optional. Leave out a link or assignment id and [`apply`]() generates one (`$generated_link_1`, `$generated_assignment_1`, ...). Generated ids come back in the report's `effects`, so you can store them in your model. ```js import { Project } from "plen-js"; const project = new Project(); project.solve({ tasks: [ { id: 1, start_date: "2026-08-10 08:00", duration: 2 }, { id: 2, start_date: "2026-08-10 08:00", duration: 3 }, ], links: [{ id: 1, source: 1, target: 2, type: "fs" }], }); const report = project.apply({ tasks: { // A new task, linked after task 2 below. add: [{ id: 3, start_date: "2026-08-10 08:00", duration: 1 }], // Task 1 now takes three days. update: [{ id: 1, duration: 3 }], }, // The link has no id: apply generates one. links: { add: [{ source: 2, target: 3, type: "fs" }] }, }); console.log(report.applied.map((op) => op.path)); console.log("generated links:", report.effects.generatedLinks); const result = project.solve(null); for (const update of result.updates) { console.log(`task ${update.id}: ${update.start_date} .. ${update.end_date}`); } ``` The report lists `tasks.add[0]`, `tasks.update[0]` and `links.add[0]`, and the new link is `$generated_link_1`. After the solve, task 1 runs until `2026-08-13 08:00`, task 2 moves to `2026-08-13 08:00 .. 2026-08-18 08:00`, and task 3 starts when task 2 ends. Operations don't run in the order you wrote them. `apply` runs them in a fixed order, so every reference comes after the adds it may need and every removal comes before the edits it could break: 1. tasks: remove, add, update 2. links: remove, update, add 3. resources: remove, add, update 4. assignments: remove, update, add An operation is named by its group, array and index, such as `tasks.update[0]`. The same path shows up in the report's `applied` and `rejected` lists. The change set itself is checked strictly: an unknown key on it or on a group throws [`InputError`](). Unknown fields inside a record are ignored, the same as in [`ProjectData`](). ## Update records Update records ([`TaskUpdate`](), [`LinkUpdate`, `ResourceUpdate`, `AssignmentUpdate`]()) name the record by `id`. Every other field is optional, and each field has three states: - Left out (or `undefined`): the field keeps its value. - A value: the field is set. - `null`: an optional field is cleared, or a field that always has a value goes back to its default. `auto_scheduling` goes back to `true`, `unscheduled` to `false`, `progress` to `0`, `priority` to `500`, `leveling_delay` to `0`, `pauses` to empty, a link's `type` to `"fs"` and its `lag` to `0`. `constraint_type: ""` also clears a constraint. ```js import { Project } from "plen-js"; const project = new Project(); project.solve({ config: { apply_constraints: true }, tasks: [{ id: "review", start_date: "2026-08-10 08:00", duration: 1 }], }); // A value sets a field: "start no earlier than Thursday". project.apply({ tasks: { update: [{ id: "review", constraint_type: "snet", constraint_date: "2026-08-13 08:00" }], }, }); for (const update of project.solve(null).updates) { console.log(`pinned: ${update.id} starts ${update.start_date}`); } // null clears the constraint; start_date moves the task back. // Every field you leave out keeps its value. project.apply({ tasks: { update: [ { id: "review", constraint_type: null, constraint_date: null, start_date: "2026-08-10 08:00" }, ], }, }); for (const update of project.solve(null).updates) { console.log(`unpinned: ${update.id} starts ${update.start_date}`); } ``` The first solve moves the task to `2026-08-13 08:00`. The second puts it back on Monday. Clearing the constraint alone wouldn't do that: the accepted state already holds the Thursday dates, so the update also sets `start_date`. ### Which dates a task update keeps A task has three linked date fields: `start_date`, `end_date` and `duration`. When an update sets some of them, [`apply`]() keeps what you sent and derives the rest: - `start_date` alone keeps the duration and recomputes the end (a drag). - `end_date` alone keeps the start and recomputes the duration (a resize from the right edge). - `duration` alone keeps the start and recomputes the end. - `start_date` and `end_date` together recompute the duration. - A new `calendar_id`, or a new `parent` while calendars are inherited (`calendar_mode: "task-inherited"`), keeps the start and duration and recomputes the end on the new calendar. Under an inherited calendar, the descendants that inherit it re-derive their ends too. An explicit `start_date` on non-working time is kept as sent when the task has no predecessor. The end is computed forward from it on the working calendar, so only the start sits outside working time. Snap the date in your UI if you don't want that. ### Feeding results back An entry in a result's `updates` uses the same field names as a [`TaskUpdate`](). Its extra `kind` and `reason` fields are ignored like any unknown field, so you can pass the entries to `apply` as they are. That's useful when you want to commit dates you got from [`preview`]() or from a leveled clone: ```js import { Project } from "plen-js"; const project = new Project(); const loaded = project.solve({ tasks: [{ id: 1, start_date: "2026-08-10 08:00", duration: 2 }], }); const preview = project.preview({ tasks: { update: [{ id: 1, start_date: "2026-08-12 08:00" }] }, }); // The user confirms: commit exactly the previewed dates. project.apply({ baselineId: loaded.baselineId, tasks: { update: preview.updates } }); console.log(project.solve(null).updates); ``` The solve reports task 1 at `2026-08-12 08:00 .. 2026-08-14 08:00`, the dates the preview showed. `pauses` replaces the whole list. Set it to `[]` or `null` to remove every pause. See [Clear leveling](). ## Applying [`apply`]() returns a [report]() with these fields: - `applied` lists the path of every accepted operation. - `rejected` lists the path and message of every refused one. - `effects` lists what the edit changed beyond the records you named (below). - `needsResync` is `true` when anything was rejected. - `revision` is the project's revision number after this edit. Removals cascade, and [`effects`]() tells you what went with them. Removing a task removes its descendants, every link that touches any of them and every assignment on them. Removing a resource removes its assignments. ```js import { Project } from "plen-js"; const project = new Project(); project.solve({ tasks: [ { id: "phase", type: "summary" }, { id: "spec", parent: "phase", start_date: "2026-08-10 08:00", duration: 2 }, { id: "release", start_date: "2026-08-10 08:00", duration: 1 }, ], links: [{ id: "spec-release", source: "spec", target: "release" }], resources: [{ id: "ann" }], assignments: [{ id: "ann-spec", task: "spec", resource: "ann", value: 1 }], }); const { effects } = project.apply({ tasks: { remove: ["phase"] } }); console.log("tasks:", effects.removedTasks); console.log("links:", effects.removedLinks); console.log("assignments:", effects.removedAssignments); ``` Removing the summary `phase` also removes `spec`, the link `spec-release` and the assignment `ann-spec`. Delete the same records from your own model. The other `effects` lists: - `generatedLinks` and `generatedAssignments` hold the ids `apply` made up for additions without one. - `droppedLinks` and `droppedAssignments` hold records that were accepted but are now inactive, for example a link to an unscheduled task. They stay in the project and come back into play when the task does. `apply` edits the project in place. Clones made earlier don't see the edit, so you can [`clone()`]() a project, apply a multi-step edit to the clone and drop it if the user cancels. For a single edit, [`preview`]() does the same job without a clone. See [Previewing changes](). :::note `clone()` copies the whole project at once, so its cost grows with the project's size. ::: Field-only task edits and assignment updates take a fast path that patches the affected records without rebuilding the whole model. Frequent small edits, like drags, stay cheap. ## Rescheduling After [`apply`](), call [`solve(null)`](). It schedules the edited state, accepts the dates as the new baseline and returns the updates relative to the previous one. Tasks that a successful operation added or updated are always reported, even when the planner leaves them exactly where the edit put them. The same goes for tasks whose working calendars changed because of a resource or assignment edit. Your model gets the derived end or duration for every task you edited, not only for the ones that moved. These reported tasks accumulate across several `apply` calls until the next acyclic `solve(null)` or `level`. A cyclic solve keeps them pending. ### Guarding against stale edits A UI usually edits the dates it showed the user. If the project changed in between (an edit from another tab, a leveling run), applying the edit can do the wrong thing. Set `baselineId` on the change set to the token of the result the user saw. [`apply`]() then refuses the whole change set with [`ProjectError`]() code `"stale-baseline"` when that token isn't current. ```js import { Project, ProjectError } from "plen-js"; const project = new Project(); const loaded = project.solve({ tasks: [{ id: 1, start_date: "2026-08-10 08:00", duration: 2 }], }); const seen = loaded.baselineId; // The edit was made against the dates the user saw. const changes = { baselineId: seen, tasks: { update: [{ id: 1, duration: 3 }] } }; project.apply(changes); // A second edit still carrying the old token is refused as a whole. try { project.apply(changes); } catch (error) { if (error instanceof ProjectError && error.code === "stale-baseline") { console.log("stale: reload first"); } else { throw error; } } const solved = project.solve(null); console.log("new baseline differs:", solved.baselineId !== seen); ``` A token names a baseline only while the project is solved and clean. After an `apply`, any `baselineId` is stale until the next solve, so take a fresh token from each result. Tokens are opaque: don't store them or compare them across runs. Leave `baselineId` out to skip the check. ## Rejections and resync An operation is rejected when it can't apply: an unknown id in `update` or `remove`, a duplicate id in `add`, `null` on a required field (a link's `source` or `target`, an assignment's `task`, `resource` or `value`), or a record that fails validation after the edit, such as an invalid pause or assignment slice. Each operation is tried on its own copy, so a rejected operation leaves no partial edit behind, and the operations around it still apply. ```js import { Project, ProjectError } from "plen-js"; function load() { return { tasks: [{ id: "design", start_date: "2026-08-10 08:00", duration: 2 }] }; } const project = new Project(); project.solve(load()); const report = project.apply({ tasks: { remove: ["deleted-elsewhere"], update: [{ id: "design", duration: 4 }], }, }); for (const op of report.applied) console.log(`applied ${op.path}`); for (const op of report.rejected) console.log(`rejected ${op.path}: ${op.message}`); if (report.needsResync) { // solve(null) and apply now throw until the full data is sent again. try { project.solve(null); } catch (error) { if (!(error instanceof ProjectError && error.code === "needs-resync")) throw error; project.solve(load()); } } ``` The update applies and the removal is rejected with `rejected tasks.remove[0]: unknown task id`. The project now holds the update but not the removal, which is probably not what your model holds. That's why any rejection blocks further edits: `solve(null)`, `apply` and `level` throw [`ProjectError`]() with code `"needs-resync"` until a successful `solve(data)` replaces the whole state with data you know is right. The resync data must pass validation. If it doesn't, `solve(data)` throws [`InputError`](), and the project still needs a resync. See [Recover from rejected changes](). [`evaluate`]() and [`suggestAssignments`]() use the same apply on disposable copies. A rejected candidate shows up as its [`error`]() (`{ path, message }`) instead of blocking the project. See [Evaluating alternatives](). ## What change sets can't do Some edits have no operation. For these, rebuild the data in your model and send it with [`solve(data)`](): - Calendars can't be added, changed or removed. Tasks and resources can switch to another existing calendar through `calendar_id`. - [`config`]() can't change: `duration_unit`, `use_calendars`, `calendar_mode`, `apply_constraints` and the other settings are fixed until the next full solve. - A task's `type` and `duration_unit` aren't in [`TaskUpdate`](). An update that carries them is accepted, but the fields are ignored. To turn a task into a summary or a milestone, remove it and add it again (its links and assignments go with the removal), or resync. - An id can't be renamed. `update` finds the record by its id and never changes it. - [`ResourceUpdate`]() changes only `capacity` and `calendar_id`. Calls that don't need a change set also don't use one. [`level`]() writes its own `leveling_delay` and `pauses` edits and accepts the result. See [Leveling resources]().