Converter functions for mapping object values

mappings.ts

export type MappingRule = (data: any) => any;
export type MappingRules<T> = { [P in keyof T]?: MappingRule };
export type MapperFunction<T> = (input: T) => any;

// Maps property values based on rules that you supplied.
// Properties for which no rule was supplied are copied to the output.
// NOTICE: This mapper function does not support renaming properties.
export function mapObject<T>(input: T, rules: MappingRules<T>): any {
    if(input === undefined || input === null) {
        return input;
    }
    
    let result: any = {};

    for(let key in input) {
        
        if(input.hasOwnProperty(key)) {
            if(input[key] === undefined || input[key] === null) {
                result[key] = input[key];
            } else if(rules[key]) {
                result[key] = rules[key](input[key]);
            } else {
                result[key] = input[key];
            }
        }
    }

    return result;
}

// Generates a mapping function that converts property values based on rules.
export function mapper<T>(rules: MappingRules<T>): MapperFunction<T> {
    return (data) => mapObject(data, rules);
}

// Generates a mapping function that maps an array of objects
export function collectionMapper<T>(rules: MappingRules<T>): MapperFunction<T[]> {
    return (data) => data.map(item => mapObject(item, rules));
}