/**
 * 给对象数组按指定键值分组
 * @example
 * let arr = [{type:'a'}, {type:'a'}, {type:'b'}]
 * groupArray(arr) // => {a:[{type:'a'},{type:'a'}], b:[{type:'b'}]}
 */
export function groupArray<T>(arr: T[], key: string): { [group: string]: T[] } {
    let map: any = {}
    arr.forEach((item: any) => {
        let value = item[key]

        if (typeof value !== "string") {
            if (value === undefined) value = "undefined"
            if (value === null) value = "null"
            if (value === false) value = "false"
            if (value === true) value = "true"
        }

        let group = map[value]
        if (!group) {
            group = []
            map[value] = group
        }
        group.push(item)
    })

    return map
}