@mapbox/mapbox-gl-style-spec/util/get_type.ts

a specification for mapbox gl styles

export default function getType(val: unknown): string {
    if (val instanceof Number) {
        return 'number';
    } else if (val instanceof String) {
        return 'string';
    } else if (val instanceof Boolean) {
        return 'boolean';
    } else if (Array.isArray(val)) {
        return 'array';
    } else if (val === null) {
        return 'null';
    } else {
        return typeof val;
    }
}