> Part of the walkerOS documentation. Project overview and full index: # Mapping.Value `Mapping.Value` is what you put into any mapping field that expects a value: `data`, every entry under `map`, entries in `set[]`, keys under `policy`, and anywhere a destination rule field accepts `any`. It is polymorphic: a value can be a simple string path, an object with one of several transforming keys, or an array of those. The shape is defined in [`packages/core/src/types/mapping.ts`](https://github.com/elbwalker/walkerOS/blob/main/packages/core/src/types/mapping.ts) as `Value = ValueType | Array`, where `ValueType` is either a string (path) or a `ValueConfig` object. ## Options[​](#options "Direct link to Options") | Form | Shape | What / When | | ----------------------- | ------------------------------- | ----------------------------------------------------------------------------------------------- | | [Path](#path) | `string` | Reads a dot-path from the event. Use for copying raw fields straight into the output. | | [Constant](#constant) | `{ value }` | Emits a fixed value. Use when the destination needs a literal like a currency, ID, or label. | | [Map](#map) | `{ map: { key: Value, … } }` | Builds an object from multiple paths. Use when the destination expects a structured payload. | | [Loop](#loop) | `{ loop: [source, Value] }` | Iterates an array source and transforms each item. Use for nested product, cart, or item lists. | | [Set](#set) | `{ set: [Value, Value, …] }` | Picks the first defined value from a list. Use as a fallback chain like email then user\_id. | | [Fn](#fn) | `{ fn: (value, context) => … }` | Runs a custom function on the event. Use only when no declarative form fits. | | [Condition](#condition) | `{ condition, value }` | Gates the value on a predicate. Use for branching logic inside a single rule. | | [Consent](#consent) | `{ consent: {…}, value }` | Gates the value on consent states. Use to make individual fields privacy-aware. | All `ValueConfig` keys can be combined on the same object: `key`, `condition`, `consent`, `validate` act as filters; `value`, `fn`, `map`, `loop`, `set` produce output. ## Path[​](#path "Direct link to Path") Extract a value by dot-path from the event. Configuration Loading... Result Loading... Paths also index into arrays: Configuration Loading... Result Loading... ## Constant[​](#constant "Direct link to Constant") Return a fixed value, independent of the event. Configuration Loading... Result Loading... ## Map[​](#map "Direct link to Map") Build an object by mapping keys to their own `Mapping.Value`. Configuration Loading... Result Loading... ## Loop[​](#loop "Direct link to Loop") Iterate an array source and transform each element. `loop: [source, value]`: `source` selects the array (a path, or `'this'` for the event itself), `value` transforms each item. Configuration Loading... Result Loading... ## Set[​](#set "Direct link to Set") Try each value in order and return the first defined result. Useful as a fallback chain. ``` { set: ['data.email', 'user.email', { value: 'anonymous' }] } ``` ## Fn[​](#fn "Direct link to Fn") Run a custom function. The handler receives `(value, context)` and may return a promise. All three callbacks (`fn`, `condition`, `validate`) share the same shape. ### Context object[​](#context-object "Direct link to Context object") | Field | Type | Required | Description | | ----------- | ------------------------------- | -------- | ------------------------------------------ | | `event` | `WalkerOS.DeepPartialEvent` | yes | The root event being mapped | | `mapping` | `Mapping.Value \| Mapping.Rule` | yes | The surrounding mapping config (or rule) | | `collector` | `Collector.Instance` | yes | Active collector, use for `push` and queue | | `logger` | `Logger.Instance` | yes | Use for `info`/`warn`/`error`/`debug` | | `consent` | `WalkerOS.Consent` (optional) | no | Resolved consent at this evaluation point | One-arg callbacks like `(value) => value.toUpperCase()` continue to work, TypeScript ignores the unused `context` argument. Configuration Loading... Result Loading... ## Condition[​](#condition "Direct link to Condition") Only produce output when `condition(event)` returns truthy. ``` { condition: (event) => event.data?.value > 50, value: 'high_value', } ``` ## Consent[​](#consent "Direct link to Consent") Only produce output when the required consent states are granted on the event. Configuration Loading... Result Loading... ## Validate[​](#validate "Direct link to Validate") `validate` is a filter, not a form. Combine it with any value-producing key. The function runs on the produced value; if it returns falsy the result is dropped (`undefined`). Configuration Loading... Result Loading... ## Policy[​](#policy "Direct link to Policy") A policy is a record of `Mapping.Value`s applied to the event before the rule runs. The key is the path the result writes into; the value is any form above. **Config-level policy** applies to every event handled by the source or destination: ``` { config: { policy: { 'user.email': { fn: (e) => e.user?.email?.toLowerCase() }, 'data.timestamp': { fn: () => Date.now() }, }, }, } ``` **Event-level policy** applies only inside the matched rule: ``` { config: { mapping: { product: { view: { name: 'view_item', policy: { 'data.id': { fn: (e) => `PRODUCT_${e.data?.id}` }, }, }, }, }, }, } ``` Processing order: config policy → event matching → event policy → data transformation. ## See also[​](#see-also "Direct link to See also") * [Mapping.Rule](https://www.walkeros.io/docs/mapping/rule.md): the rule object that composes values * [`getMappingValue` API](https://www.walkeros.io/docs/mapping/rule.md#getmappingvalue): programmatic value resolution * [Consent guide](https://www.walkeros.io/docs/guides/consent.md): consent model in depth