# How to create reusable formula blocks in MakeCode extensions?

**URL:** <https://forum.makecode.com/t/how-to-create-reusable-formula-blocks-in-makecode-extensions/45112>\
**Category:** Help\
**Tags:** graphics-and-math, extension\
**Created:** [July 28, 2026, 9:02pm UTC](https://forum.makecode.com/t/how-to-create-reusable-formula-blocks-in-makecode-extensions/45112 "2026-07-28T21:02:37Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![CodaKnight](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.makecode.com/codaknight/32/33491_2.png) [@CodaKnight](https://forum.makecode.com/u/CodaKnight)\
**Post date:** [July 28, 2026, 9:02pm UTC](https://forum.makecode.com/t/how-to-create-reusable-formula-blocks-in-makecode-extensions/45112/1 "2026-07-28T21:02:37Z")

</div>

Hi @richard ! I had a question about custom blocks in extensions.

I’m working on a MathX extension, and I’d like users to be able to define reusable formulas in Blocks, something conceptually like:

```auto
define formula "square" with input x
    x × x

```

Then later evaluate it with something like:

```auto
evaluate formula "square" with 5

```

I tried using callback handlers (`handlerStatement`, `draggableParameters`, etc.), but I ran into the issue that the callback only provides the evaluated value and not the expression itself, so I can’t store the formula for later evaluation.

Is there a supported way for an extension to create a block like this, or would it require custom Blockly/PXT changes? If it isn’t currently possible, is there a recommended approach for implementing something similar in a MakeCode extension?

Thanks!

Code:

```typescript
// CUSTOM FUNCTIONS 

    type FormulaFn = (x: number) => number

    let formulas: { [key: string]: FormulaFn } = {}

    let currentFormula = ""

    let currentInput = 0

    /**

    * Define a formula

    */

    //% blockId=mathx_define_formula

    //% block="define formula $name with input $x"

    //% handlerStatement=1

    //% draggableParameters="reporter"

    //% group="Create"

    //% subcategory="Custom Functions"

    //% name.shadow="text"

    //% x.defl="x"

    export function defineFormula(name: string, handler: (x: number) => void): void {

        currentFormula = name

        // Run once so "set result" executes while defining.

        handler(0)

        currentFormula = ""

    }

    /**

     * Formula input

     */

    //% blockId=mathx_formula_input

    //% block="input"

    //% group="Create"

    //% subcategory="Custom Functions"

    export function input(): number {

        return currentInput

    }

    /**

    * Set formula result

    */

    //% blockId=mathx_set_formula_result

    //% block="set result to $value"

    //% group="Create"

    //% subcategory="Custom Functions"

    //% value.shadow="math_number"

    export function setFormulaResult(value: number): void {

        if (!currentFormula)

            return

        formulas[currentFormula] = (x: number) => {

            currentInput = x

            return value

        }

    }

    /**

    * Evaluate formula

    */

    //% blockId=mathx_eval_formula

    //% block="evaluate formula $name with $x"

    //% group="Run"

    //% subcategory="Custom Functions"

    //% name.shadow="text"

    //% x.shadow="math_number"

    export function evalFormula(name: string, x: number): number {

        let fn = formulas[name]

        if (!fn)

            return 0

        return fn(x)

    }

```

Github: (I haven’t pushed the changes yet for it)

> **[GitHub - CrzLe0723/MathX: MathX is a utility extension for MakeCode Arcade...](https://github.com/CrzLe0723/MathX)**
>
> MathX is a utility extension for MakeCode Arcade that includes advanced mathematical functions, vector operations, interpolation, easing, movement helpers, timing utilities, and physics calculations to make game development easier.

---

<div class="post-metadata">

**Author:** ![richard](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.makecode.com/richard/32/5417_2.png) [@richard](https://forum.makecode.com/u/richard)\
**Post date:** [July 29, 2026, 5:31pm UTC](https://forum.makecode.com/t/how-to-create-reusable-formula-blocks-in-makecode-extensions/45112/2 "2026-07-29T17:31:38Z")

</div>

sure, i’ve done this thing in a few of my extensions (most notably [arcade-tile-scanner](https://github.com/riknoll/arcade-tile-scanner) and [arcade-rpg](https://github.com/riknoll/arcade-rpg))

there is no way to define callbacks with return values in blocks, but you can fake it by building up an equation using functions you create. here’s an example of what I mean:

> **[equations\_example](https://arcade.makecode.com/S57952-32942-91955-34292)**
>
> Made with ❤️ in Microsoft MakeCode Arcade.

the blocks code is in custom.ts

the downside to this approach is that you basically have to remake all of the math blocks and people need to know to use your versions instead of the ones in the math category.

---

<div class="post-metadata">

**Author:** ![CodaKnight](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.makecode.com/codaknight/32/33491_2.png) [@CodaKnight](https://forum.makecode.com/u/CodaKnight)\
**Post date:** [July 30, 2026, 2:28am UTC](https://forum.makecode.com/t/how-to-create-reusable-formula-blocks-in-makecode-extensions/45112/3 "2026-07-30T02:28:16Z")

</div>

> [@richard](#):
>
> the downside to this approach is that you basically have to remake all of the math blocks and people need to know to use your versions instead of the ones in the math category.

That makes sense. I hadn’t considered that I’d need to recreate most of the math blocks. I think I’ll experiment with it and see whether the usability is worth the extra blocks.