---
title: MARKLESS_EVENT_HANDLER_NOT_A_FUNCTION
description: "An event prop like onClick gets a value instead of a function."
sidebar: { hidden: true }
search: { keywords: ["EVENT_HANDLER_NOT_A_FUNCTION","event handler not a function","markless error"] }
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---

**Error** from the compiler, while it builds your `.tsrx` files.

## What happened

An event prop needs a function. This one gets the result of an expression, which runs once during render.

Markless prints the message, the reason, and a suggestion:

```text
MARKLESS_EVENT_HANDLER_NOT_A_FUNCTION: `<attributeName>={<source>}` passes the result of `<source>`, not a function. The expression would run once while rendering, and the click would receive a number.
An event prop compiles to a lazy handler symbol that runs on the browser event; only a function can be that handler.
Wrap it in a function, for example <attributeName>={() => <source>}.
https://markless.dev/errors/MARKLESS_EVENT_HANDLER_NOT_A_FUNCTION
```

Words in angle brackets change from case to case.

## How to fix it

Wrap the expression in an arrow function.

```tsrx
<button onClick={() => count++}>Add</button>
```

## Where it comes from

`@markless/compiler` raises this code. Source:

- [packages/compiler/src/passes/semantic-graph/collect-elements.ts](https://github.com/compiled-run/markless/blob/main/packages/compiler/src/passes/semantic-graph/collect-elements.ts)

Read [how to read a Markless error](/tooling/diagnostics), or go back to [all error codes](/errors).