How to define css variables in style attribute in React and typescript

Question

I want to define jsx like this:

<table style={{'--length': array.lenght}}>
   <tbody>
      <tr>{array}</tr>
   </tbody>
</table>

and I use --length in CSS, I also have cells that have --count that shows count using CSS pseudo selector (using the counter hack).

but typescript throws error:

TS2326: Types of property 'style' are incompatible.
  Type '{ '--length': number; }' is not assignable to type 'CSSProperties'.
    Object literal may only specify known properties, and ''--length'' does not exist in type 'CSSProperties'.

is it possible to change type of style attribute to accept CSS variable (custom properties) or is there a way to force any on style object?

Answer 1

Like this:

function Component() {
  const style = { "--my-css-var": 10 } as React.CSSProperties;
  return <div style={style}>...</div>
}

Or without the extra style variable:

function Component() {
  return <div style={{ "--my-css-var": 10 } as React.CSSProperties} />
}

Answer 2

If you go to the definition of CSSProperties, you'll see:

export interface CSSProperties extends CSS.Properties<string | number> {
    /**
     * The index signature was removed to enable closed typing for style
     * using CSSType. You're able to use type assertion or module augmentation
     * to add properties or an index signature of your own.
     *
     * For examples and more information, visit:
     * https://github.com/frenic/csstype#what-should-i-do-when-i-get-type-errors
     */
}

That link gives examples of how to solve the type error by augmenting the definition of Properties in csstype or casting the property name to any.

Answer 3

You can add a type assertion to the variable. i.e. {['--css-variable' as any]: value }

<table style={{['--length' as any]: array.length}}>
   <tbody>
      <tr>{array}</tr>
   </tbody>
</table>

Answer 4

Casting the style to any defeats the whole purpose of using TypeScript, so I recommend extending React.CSSProperties with your custom set of properties:

import React, {CSSProperties} from 'react';

export interface MyCustomCSS extends CSSProperties {
  '--length': number;
}

By extending React.CSSProperties, you will keep TypeScript's property checking alive and you will be allowed to use your custom --length property.

Using MyCustomCSS would look like this:

const MyComponent: React.FC = (): JSX.Element => {
  return (
    <input
      style={
        {
          '--length': 300,
        } as MyCustomCSS
      }
    />
  );
};