JavaScript function to categorize data by their "styles" attribute, and store the key-"unicode" pair
First off, some code review items;


Don't skip on semicolons
You are not changing a number of variables, consider const over let in those cases
The names of your styles match the names of your variables, you could use this


Given that, I would consider something like this

function processMetadata(metadata) {
    let out = {regular: {}, solid: {}, brands: {}};
    for (const icon in metadata) {
        const styles = metadata[icon].styles;
        const codePoint = metadata[icon].unicode;
        for(const style in out){
            if (styles.includes(style)) {
                out[style][icon] = codePoint;
            }
        }
    }
    return out;
}