TypeScript 2.4.1 -> fontWeight -> Type 'number' is not assignable to type '"inherit" | 400 |

Question

I get this error when I try to set fontWeight in TypeScript:

Types of property 'test' are incompatible.
    Type '{ fontWeight: number; }' is not assignable to type 'Partial<CSSProperties>'.
        Types of property 'fontWeight' are incompatible.
            Type 'number' is not assignable to type '"inherit" | 400 | "initial" | "unset" | "normal" | "bold" | "bolder" | "lighter" | 100 | 200 | 30...'.

Even though 400 is a correct number it could be any number and therefore I get the error as I understand it. I can track this error to React.CSSProperties that specifies that fontWeight should look like this:

fontWeight?: CSSWideKeyword | "normal" | "bold" | "bolder" | "lighter" | 100 | 200 | 300 | 400 | 500 | 600 | 700 | 800 | 900;

What I can't do is set test: React.CSSProperties

const styles = (theme: Theme) => ({
    test: {
        fontWeight: 400
    }
});

I can do it like this but it is not how Material UI handles classes.

const test: React.CSSProperties = {
    fontWeight: 400
}

Complete code:

import * as React from "react";
import * as ReactDOM from "react-dom";
import * as ReactRouter from "react-router";
import { withRouter } from "react-router-dom";
import { withStyles } from 'material-ui/styles';
import Badge from 'material-ui/Badge';
import Grid from 'material-ui/Grid';
import { Theme } from 'material-ui/styles';

interface IState {
    userName: string;
}

interface IProps {
    history?: any;
    classes?: any;
}

const styles = (theme: Theme) => ({
    test: {
        fontWeight: 400
    }
});

class Menu extends React.Component<IProps, IState> {
    constructor(props: IProps) {
        super(props);
        this.state = {
            userName: localStorage.userName ? 'userName ' + localStorage.userName : "",
        }
    }
    render() {
        return (
            <div>
                <Grid container spacing={24}>
                    <Grid item xs={12} className={this.props.classes.test}>
                    <span>Test</test>
                    </Grid>
                </Grid>
            </div>
        );
    }
}

Answer 1

Currently accepted answer for this question is just a workaround. The proper way to solve this problem is using createStyles function from Material-UI.

const styles = (theme: Theme) => createStyles({
    test: {
        fontWeight: 400
    }
});

See the material UI documentation.

Answer 2

Solved it:

const styles = (theme: Theme) => ({
    test: {
        fontWeight: 400
    } as React.CSSProperties
});