lit-css-vars

For easily creating and sharing typed CSS vars for the lit ecosystem.

Installation

npm i lit-css-vars

Usage

import {css} from 'lit';
import {defineCssVars} from 'lit-css-vars';

// css vars definition
export const myVars = defineCssVars({
    // key is CSS var name
    'my-var-name': 'blue', // value is the CSS var's default value
});

// usage
function renderStyles() {
    return css`
        p {
            /*
                This sets the CSS var's value to red. This works because ".name" is "--my-var-name".
            */
            ${myVars['my-var-name'].name}: red;
        }

        span {
            /*
                This shows how to use the CSS var's value. If a span is within a <p> element, color
                will be set to red. If not, the default value of blue (defined earlier) will be
                applied. This works because ".value" is "var(--my-var-name, blue)".
            */
            color: ${myVars['my-var-name'].value};
        }
    `;
}

Errors

If you see the following error messages in your types, read the explanations below on how to fix them.

Error: input CSS var names are too generic.

This happens if your input to createCssVars is too vague. This means that specific var names can't be extracted from the input object. This may happen if your input object has the vague key type of just string, like Record<string, string>. You need to make sure you use as const or somehow prevent TypeScript from broadening your input type.

import {CssVarsSetup, defineCssVars} from 'lit-css-vars';

/**
 * This fails because assigning the object to type CssVarsSetup kills the specific 'my-var-name' key
 * name and instead assigns the object a generic CssVarName key (the key requirement for
 * CssVarsSetup).
 */
const BAD_VARS_SETUP_DO_NOT_USE: CssVarsSetup = {
    'my-var-name': 'blue',
};

// @ts-expect-error: this errors because defineCssVars knows its input is too broad
export const BAD_VARS_DO_NOT_USE = defineCssVars(BAD_VARS_SETUP_DO_NOT_USE);

/**
 * This example works wonderfully because defineCssVars does not broaden the input type but still
 * holds it to a specific type requirement.
 */
export const myGoodVars = defineCssVars({
    'my-var-name': 'blue',
});

/**
 * This example works by using a helper function, wrapNarrowTypeWithTypeCheck, that requires the
 * input object to conform to the passed type (CssVarsSetup) without broadening the input type.
 * TypeScript thus knows that the exact key of goodVarsSetup is 'my-var-name' and all is well with
 * defineCssVars below.
 */
const goodVarsSetup = {
    'my-var-name': 'blue',
} satisfies CssVarsSetup;
export const myGoodVars2 = defineCssVars(goodVarsSetup);

Error: all CSS var names must be lower-kebab-case.

All CSS var name keys must be in lower-kebab-case:

import {defineCssVars} from 'lit-css-vars';

// @ts-expect-error: expect errors because we're intentionally passing in invalid CSS var names as an example
export const myCssVars = defineCssVars({
    // good: kebab-lower-case
    'my-var-name': 'green',
    'my-var': 'green',
    // bad: has upper case letters
    'My-Var': 'red',
    // bad: not kebab case, must have at least one dash (-)
    thing: 'red',
});