TypeScript - typeof array of union types

Question

If I have array of union I can check for typeof value this way:

//case 1
function something1(a1: Array<number | string | boolean>)
{
    for (const v of a1)
        if (typeof v === "number")
            v;  //v is number
        else if (typeof v === "boolean")
            v;  //v is boolean
        else
            v;  //v is string
}

If I have union of array I could do it the same way:

//case 2
function something2(a1: Array<number> | Array<string> | Array<boolean>)
{
    for (const v of a1)
        if (typeof v === "number")
            v;  //v is number
        else if (typeof v === "boolean")
            v;  //v is boolean
        else
            v;  //v is string
}

but I want to avoid checking for type inside loop:

//case 3
function something3(a1: Array<number> | Array<string> | Array<boolean>)
{
    if (a1.length === 0)
        return;
    if (typeof a1[0] === "number")
        a1;  //should be number[]!!     but it is  number[] | string[] | boolean[]
    if (typeof a1[0] === "boolean")
        a1;  //should be boolean[]!!    but it is  number[] | string[] | boolean[]
    if (typeof a1[0] === "string")
        a1;  //should be string[]!!     but it is  number[] | string[] | boolean[]
}

but a1 is not recognised as number[] or string[] or boolean[].
But this makes sense. Since all items in array are the same type. Is it possible to achieve that?

I am using TypeScript beta 2.0.

Paleo · Accepted Answer · 2016-08-26 13:47:36Z

3

With type guards:

function something3(a1: Array<number> | Array<string> | Array<boolean>) {
    if (a1.length === 0)
        return;
    if (isArrayOfNumbers(a1))
        a1[0].toExponential();
    if (isArrayOfBooleans(a1))
        a1[0].valueOf();
    if (isArrayOfStrings(a1))
        a1[0].indexOf('abc');
}

function isArrayOfNumbers(arr: number[] | string[] | boolean[]) : arr is number[] {
    return arr.length > 0 && typeof arr[0] === 'number';
}

function isArrayOfBooleans(arr: number[] | string[] | boolean[]) : arr is boolean[] {
    return arr.length > 0 && typeof arr[0] === 'boolean';
}

function isArrayOfStrings(arr: number[] | string[] | boolean[]) : arr is string[] {
    return arr.length > 0 && typeof arr[0] === 'string';
}

answered Aug 26, 2016 at 13:47

Paleo

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