[add] api page and guide section on adding custom formulas

docs/api/api_overview.md

@@ -25,6 +25,7 @@ Parameters:
 | Name                                         | Description                                         |
 | :------------------------------------------- | :-------------------------------------------------- |
 | [](api/spreadsheet_addcolumn_method.md)      | @getshort(api/spreadsheet_addcolumn_method.md)      |
+| [](api/spreadsheet_addformula_method.md)     | @getshort(api/spreadsheet_addformula_method.md)     |
 | [](api/spreadsheet_addrow_method.md)         | @getshort(api/spreadsheet_addrow_method.md)         |
 | [](api/spreadsheet_addsheet_method.md)       | @getshort(api/spreadsheet_addsheet_method.md)       |
 | [](api/spreadsheet_clear_method.md)          | @getshort(api/spreadsheet_clear_method.md)          |

docs/api/overview/methods_overview.md

@@ -9,6 +9,7 @@ description: You can have a Methods overview of the DHTMLX JavaScript Spreadshee
 | Name                                        | Description                                        |
 | :------------------------------------------ | :------------------------------------------------- |
 | [](../spreadsheet_addcolumn_method.md)      | @getshort(../spreadsheet_addcolumn_method.md)      |
+| [](../spreadsheet_addformula_method.md)     | @getshort(../spreadsheet_addformula_method.md)     |
 | [](../spreadsheet_addrow_method.md)         | @getshort(../spreadsheet_addrow_method.md)         |
 | [](../spreadsheet_addsheet_method.md)       | @getshort(../spreadsheet_addsheet_method.md)       |
 | [](../spreadsheet_clear_method.md)          | @getshort(../spreadsheet_clear_method.md)          |

docs/api/spreadsheet_addformula_method.md

@@ -0,0 +1,51 @@
+---
+sidebar_label: addFormula()
+title: addFormula method
+description: You can learn about the addFormula method in the documentation of the DHTMLX JavaScript Spreadsheet library. Browse developer guides and API reference, try out code examples and live demos, and download a free 30-day evaluation version of DHTMLX Spreadsheet.
+---
+
+# addFormula()
+
+### Description
+
+@short: Registers a custom formula function that can be used in cell formulas
+
+Once registered, the formula is available in any cell by its uppercase name (e.g. =MYFUNC(A1, B2)).
+
+### Usage
+
+~~~jsx
+addFormula(name: string, handler: (...args: any[]) => any): void;
+~~~
+
+### Parameters
+
+- `name` - (*string*) required, the formula name (case-insensitive, stored as uppercase)
+- `handler` - (*(...args: any[]) => any*) required, the function that computes the formula result. Receives the resolved cell values as arguments
+
+:::note
+The `handler` callback function must be synchronous. Using `Promise` or `fetch` inside the function is not allowed.
+:::
+
+### Example
+
+~~~jsx {4-6}
+const spreadsheet = new dhx.Spreadsheet("spreadsheet_container", {});
+
+// Adds a custom formula that doubles a value
+spreadsheet.addFormula("DOUBLE", (value) => {
+    return value * 2;
+});
+
+// Now use in cells: =DOUBLE(A1)
+spreadsheet.parse([
+    { cell: "A1", value: 4, format: "number" },
+    { cell: "B1", value: "=DOUBLE(A1)", format: "number" }
+]);
+~~~
+
+**Change log:** Added in v6.0
+
+**Related sample:** [Spreadsheet. Custom formula](https://snippet.dhtmlx.com/wvxdlahp)
+
+**Related articles:** [Formulas and functions](/functions/#custom-formulas)

docs/functions.md

@@ -1326,4 +1326,31 @@ When you enter a formula, a popup with description of the function and its param
 
 Check the example in our [snippet tool](https://snippet.dhtmlx.com/wux2b35b).
 
-You can modify the default locale for the popup with formula parameters and add a custom locale. Check the details in the [Localization](localization.md/#default-locale-for-formulas) guide.
+You can modify the default locale for the popup with formula parameters and add a custom locale. Check the details in the [Localization](localization.md/#default-locale-for-formulas) guide.
+
+## Custom formulas
+
+Starting with v6.0, you can register custom formula functions via the [`addFormula()`](api/spreadsheet_addformula_method.md) method. Once registered, the formula is available in any cell by its uppercase name.
+
+The method takes two parameters: the formula name and a synchronous handler function that receives the resolved cell values as arguments and returns the result:
+
+~~~js
+spreadsheet.addFormula("DOUBLE", (value) => {
+    return value * 2;
+});
+~~~
+
+After that, the formula can be used in cells just like any built-in function:
+
+~~~js
+spreadsheet.parse([
+    { cell: "A1", value: 4, format: "number" },
+    { cell: "B1", value: "=DOUBLE(A1)", format: "number" }
+]);
+~~~
+
+:::note
+The handler function must be synchronous. Using `Promise` or `fetch` inside the function is not allowed.
+:::
+
+**Related sample:** [Spreadsheet. Custom formula](https://snippet.dhtmlx.com/wvxdlahp)

sidebars.js

@@ -48,6 +48,7 @@ module.exports = {
                     },
                     items: [
                         "api/spreadsheet_addcolumn_method",
+                        "api/spreadsheet_addformula_method",
                         "api/spreadsheet_addrow_method",
                         "api/spreadsheet_addsheet_method",
                         "api/spreadsheet_clear_method",