typescript how to use type guard for key-value

Question

type Dircory<T> {
  [key:string]: T
}

function colname(columns:Array<string> | string| Dictory<string>, name){
 if(Array.isArray(columns){
    return columns.map(column=>table_name + "." + columns);
 }else if(typeof columns == "string"){
    return table_name + "." + columns; 
 }else if(typeof columns == "object"){
    return columns as Dictory<string>
 }
}

That's my code, but the return type of colname always Dictory<string> whatever pass any type, how should I correctly use type guard for key-value

Could you provide the specific values you are passing and where the actual result differs from your expected result? — Sefe
– Sefe, Commented Nov 19, 2018 at 10:09

Titian Cernicova-Dragomir · Accepted Answer · 2018-11-19 10:11:10Z

The problem is not with the type guard. If you want the function to return a value of the same type you will either need to have multiple overloads, or a type parameter that captures the actual type of the parameter and return the same type parameter:

type Dircory<T> = {
    [key: string]: T
}

let table_name;
function colname<T extends Array<string> | string | Dircory<string>>(columns: T, name: string) :T
function colname(columns: Array<string> | string | Dircory<string>, name) {
    if (Array.isArray(columns)) {
        return columns.map(column => table_name + "." + columns);
    } else if (typeof columns == "string") {
        return table_name + "." + columns;
    } else if (typeof columns == "object") {
        return columns as Dircory<string>
    }
}

let a = colname("A", ""); // string, actually the string literal type "A", but will work with strnig vars too 
let arr = colname(["A"], ""); // string[] 
let d = colname({a: "A"}, ""); // { a: string; }

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