# A custom operation Declared per process (in each process that uses it), href relative to the process's file: ```xml ``` ```js // ops/text.js: an export named as the operation, or the default export export async function slugify(process, context, op, evt) { const name = await context.expr.evaluateText(op.getAttribute("alias"), evt); if( !name ) throw new Error("slugify: no alias attribute"); // the error step's description const out = new EVT_EventObject(); out.value = String(evt.value || "").toLowerCase().replace(/[^a-z0-9]+/g, "-").replace(/^-|-$/g, ""); context.setAlias(name, out); } ``` - `op`: a copy of the `` element. Its children's `{...}` are filled already, its own attributes are not: fill each with `await context.expr.evaluateText(op.getAttribute("x"), evt)`. `fn.noParse = 1` hands over the children unfilled. - `evt`: the `value=` reference, resolved: `{...}` text gives `evt.value`; data (`#doc#/r/@t`) gives `evt.dataDoc` and `evt.xpath`, read with `new SYS_URL(evt.dataDoc).selectSingleText(evt.xpath)`. - Write a node: `new SYS_URL(evt.dataDoc).replaceText(text, xpath, null, context)`. - `context`: the application instance: `getAlias(name)`, `setAlias(name, evt)`, `getView(name)`, `expr`. `process.error(msg)` logs. - Return a Promise (an `async` function does); the step waits for it. To fail, throw an Error as above: the step's `error=` step runs with its message (`!#/error/description`), else it is logged and the step goes on. ## Test As a function, with stubs: `slugify(process, context, el, evt)` with `el.getAttribute`, `context.expr.evaluateText` and `context.setAlias` stubbed; assert the alias, and that it rejects. In a running app: a trigger, then read `$slug`. ## In xios-core `operations/PO_.js` with `export default async function PO_(process, context, op, evt)`; an `