Add class?

Question

React

Good evening.
i can't add class via react setState.

state = {
        done: false
    };
    onLabelClick = () => {
        this.setState({
            done: true
        });
    }
let classNames = "todo-list-item";
       if (done) {
        classNames += " done"
       }

And my css looks like this

.todo-list-item done {
 text-decoration: line-through;
}

In theory, everything with the logic should be correct, I think the error is in the syntax. Please tell me exactly where.
Thank you for your reply)

Answer 1

.todo-list-item.done {
 text-decoration: line-through;
}

and it still makes sense to write like this:

state = {
        done: false,
    };
    
    onLabelClick = () => {
        this.setState({
            done: true
        });
    };

    render() {
        return (
            <div className={'todo-list-item' + (this.state.done ? ' done' : '')} onClick={this.onLabelClick}>text</div>
        );
    }