feature request - null-conditional operator '?'

[dardino]

Scenario
I have an Invoice Class with a property Customer like this:

export class Invoice {
    // [...]
    public Customer: Customer | null | undefined;
    // [...]
}
export class Customer {
    // [...]
    public MainContact: Contact | null | undefined;
    // [...]
}
export class Contact {
    // [...]
    public Name: string;
    // [...]
}

To get the Invoice.Customer.MainContact.Name I need to write something like this:

// 
function GetCliDesc(invoice: Invoice | null | undefined): string | null {
   return (invoice ?
      invoice.Customer ?
         invoice.Customer.MainContact ?
            invoice.Customer.MainContact.Name
   : undefined : undefined : undefined);
}

Proposal:
c# like null-conditional operator ?:

function GetCliDesc(invoice: Invoice | null | undefined) {
   return invoice?.Customer?.Contact?.Name;
}
// inferred return type is T | undefined
// where T is the type of last property in the chain:
function GetCliDesc(invoice: Invoice | null | undefined): string | undefined {
   return invoice?.Customer?.Contact?.Name; // ok
}
function GetCliDesc(invoice: Invoice | null | undefined): string | null {
   return invoice?.Customer?.Contact?.Name; // error with --strictNullChecks true
}
function GetCliDesc(invoice: Invoice | null | undefined): string {
   return invoice?.Customer?.Contact?.Name; // error with --strictNullChecks true
}
function GetCliDesc(invoice: Invoice | null | undefined): string {
   return invoice?.Customer?.Contact?.Name || ""; // ok
}

console.log(GetCliDesc(null)) // undefined
console.log(GetCliDesc({})) // undefined
console.log(GetCliDesc({ Customer: null })) // undefined
console.log(GetCliDesc({ Customer: {} })) // undefined
console.log(GetCliDesc({ Customer: { MainContact: null } })) // undefined
console.log(GetCliDesc({ Customer: { MainContact: {} } })) // undefined
console.log(GetCliDesc({ Customer: { MainContact: { Name: "Jhon Wick" } } })) // "Jhon Wick"

let a: string = invoice?.Customer?.Contact?.Name; // error type 'string | null' is not assignable to type 'string'
let a: string = GetCliDesc(); // error type 'string | null' is not assignable to type 'string' 

let firstName = "Jhon Wick";
console.log(firstName?.length) // 9
let firstName = "";
console.log(firstName?.length) // 0
let firstName = null;
console.log(firstName?.length) // undefined
let firstName = undefined;
console.log(firstName?.length) // undefined

the translated javascript can be something like this:

function GetCliDesc(invoice) {
   return (invoice !== undefined && invoice !== null) ?
        (invoice.Customer !== undefined && invoice.Customer !== null) ?
             (invoice.Customer.MainContact !== undefined && invoice.Customer.MainContact !== null) ?
                  invoice.Customer.MainContact.Name
                  : undefined
             : undefined
        : undefined;
}
var a = (invoice !== undefined && invoice !== null) ?
        (invoice.Customer !== undefined && invoice.Customer !== null) ?
             (invoice.Customer.MainContact !== undefined && invoice.Customer.MainContact !== null) ?
                  invoice.Customer.MainContact.Name
                  : undefined
             : undefined
        : undefined;

[krryan]

Suggest that you refer to the operator as the ?. operator. Or would you include like mightBeFunction?() in this proposal?

[dardino]

I think mightBeFunction?() can be and interesting case

[dardino]

another case can be this:

let dictionary: { [key:string]: Function } = {};
function myfunc(p: string): Function | undefined {
   return dictionary[p];
}
let a = myfunc(p1)?(p2);

[krryan]

Fair enough. Probably worth including in the proposal then, though. And I guess ?[ could also be a thing.

To me, if you have value: A | null | undefined, value?.prop should have the following mapping: (value: A) => A['prop'], (value: null) => null, (value: undefined) => undefined. Your description would have it always map to undefined, though, as far as I can tell. Checking for null vs. undefined does complicate the JavaScript output, but at least in my project, null values becoming undefined could potentially be seen as a code smell—I could see my team avoiding the use of this kind of operator for that reason. Thoughts?

[sandersn]

A null propagation operator is a proposed Ecmascript feature. Fortunately, it looks like it advanced to Stage 1 at the January meeting. Typescript tries not to propose new syntax. It implements Ecmascript standard features when they reach stage 3. In the meantime, discussion of how to type this operator happens at #16.