Encountering an issue while attempting to initiate a nested array: "Cannot assign a value to an optional property access in the left-hand side of an assignment expression."

Question

Encountering an issue while attempting to initiate a nested array: "Cannot assign a value to an optional property access in the left-hand side of an assignment expression."

I am dealing with an object that contains nested arrays, structured like this:

export class OrdenCompra {
public id?: number,
public insumos?: OrdenCompraInsumo[],
}

export class OrdenCompraInsumo {
id?: number;
traslados?: IImpuestoTraslado[];
}

export class ImpuestoTraslado{
public id?: number,
public impuesto?: number
}

I am trying to add a value to the array named

traslados

as shown below

const retencion = new ImpuestoRetencion();
ordenCompra?.insumos[index]?.retenciones?.push(retencion);

However, the issue is that at this point,

ordenCompra?.insumos[index]?.retenciones?

it is currently

undefined

resulting in the value not being assigned. I have attempted to initialize it but keep encountering errors, such as:

ordenCompra?.insumos[index]?.retenciones = []

or

ordenCompra?.insumos[index]?.retenciones = ImpuestoRetencion[];

or

ordenCompra?.insumos[index]?.retenciones: ImpuestoRetencion[] || [];

Each attempt returns the error message:

The left-hand side of an assignment expression may not be an optional property access.

As a result, I have been unable to successfully assign a value to this array. While I understand this may seem like a basic question, I have spent hours searching for a solution without success.

arrays angular typescript

Answer 1

Answer №1

?. is also known as optional chaining and it primarily focuses on reading/invoking rather than setting values. According to the documentation:

Essentially, optional chaining enables us to create code where TypeScript can immediately cease running certain expressions when encountering a null or undefined value.

For instance, you could interpret const foobarbaz = foo?.bar?.baz like this:

// Will be of type: undefined | typeof baz
const foobarbaz = foo === undefined || foo === null
                    ? undefined
                    : foo.bar === undefined || foo.bar === null
                      ? undefined
                      : foo.bar.baz

In the context of assignment, it doesn't make sense because you cannot assign null or undefined to a value:

foo?.bar?.baz = foobarbaz
// ^^^^^^^^^^ - The left-hand side of an assignment expression may not be
//                an optional property access.

// Essentially equivalent to:
(foo === undefined || foo === null
  ? undefined
  : foo.bar === undefined || foo.bar === null
    ? undefined
    : foo.bar.baz) = foobarbaz

To assign a value to this property, you need to ensure that the value of foo.bar.baz is not nullish. This can be achieved by enclosing the assignment in an if statement:

foo.bar.baz = something; // error!

if(foo?.bar?.baz) {
  foo.bar.baz = something; // OK
}

If you have certainty that your value isn't null or undefined but the compiler isn't able to infer this automatically, consider using the non-null assertion operator (!.):

foo!.bar!.baz = something; // OK

Check out this playground illustrating each of these scenarios.

Answer 2

?. is also known as optional chaining and it primarily focuses on reading/invoking rather than setting values. According to the documentation:

Essentially, optional chaining enables us to create code where TypeScript can immediately cease running certain expressions when encountering a null or undefined value.

For instance, you could interpret const foobarbaz = foo?.bar?.baz like this:

// Will be of type: undefined | typeof baz
const foobarbaz = foo === undefined || foo === null
                    ? undefined
                    : foo.bar === undefined || foo.bar === null
                      ? undefined
                      : foo.bar.baz

In the context of assignment, it doesn't make sense because you cannot assign null or undefined to a value:

foo?.bar?.baz = foobarbaz
// ^^^^^^^^^^ - The left-hand side of an assignment expression may not be
//                an optional property access.

// Essentially equivalent to:
(foo === undefined || foo === null
  ? undefined
  : foo.bar === undefined || foo.bar === null
    ? undefined
    : foo.bar.baz) = foobarbaz

To assign a value to this property, you need to ensure that the value of foo.bar.baz is not nullish. This can be achieved by enclosing the assignment in an if statement:

foo.bar.baz = something; // error!

if(foo?.bar?.baz) {
  foo.bar.baz = something; // OK
}

If you have certainty that your value isn't null or undefined but the compiler isn't able to infer this automatically, consider using the non-null assertion operator (!.):

foo!.bar!.baz = something; // OK

Check out this playground illustrating each of these scenarios.

Encountering an issue while attempting to initiate a nested array: "Cannot assign a value to an optional property access in the left-hand side of an assignment expression."

Answer №1

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