Unable to run Array.map on an Array

[xeoneux]

TypeScript Version:

1.8

Code

function getARPTable() {
    return new Promise((resolve, reject) => {
        arp.table((error, devices) => {
            if (error) reject(error);
            else resolve(devices);
        });
    });
}

async function scanPorts() {
    let devices = await getARPTable();
    console.log(Array.isArray(devices)); // true
    devices = devices.map((device) => device); // error
                      ^
    return true;
}

scanPorts();

Expected behavior:

No error output

Actual behavior:

error TS2339: Property 'map' does not exist on type '{}'.

[kitsonk]

TypeScript cannot contextually determine what the Promise resolves to (see: #5254) therefore you have to assert it.

Something like:

function getARPTable() {
    return new Promise<any[]>((resolve, reject) => {
        arp.table((error, devices) => {
            if (error) reject(error);
            else resolve(devices);
        });
    });
}

[xeoneux]

@kitsonk Thanks! I created a similar interface to fix it. However, I am still not sure why does the error occur even after successful compilation to JavaScript and running under node?

[kitsonk]

Because, instead of inferring type any for the Promise resolution, TypeScript infers {} which is an object with no properties, therefore doesn't have a .map method. IIRC there was a lot of discussion in the ticket I linked to as to why {} instead of any makes sense when filling a generic that cannot be contextually determined. By default, TypeScript will still emit the code, even if it has errors. So while the run-time behaviour works, TypeScript couldn't be sure it would be when it emitted the code.

[xeoneux]

@kitsonk I didn't realise that the error was during compilation and not during runtime. Thanks for the clarification! 😄