can not read Property 'map' of Undefined

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I’m trying to create some dynamic checkboxes, but when I run the browser informs

"ERROR Typeerror: Cannot read Property 'map' of Undefined"

Because?

export class FormPostsComponent implements OnInit {

form: FormGroup;
categories: Category[];

constructor(
    private categoriesService: CategoriesService,
    private formBuilder: FormBuilder
) { }

ngOnInit() {

    this.categoriesService.read().subscribe(categories => this.categories = categories);

    this.form = this.formBuilder.group({
        id: [null],
        title: [null, [Validators.required, Validators.minLength(3)]],
        body: [null, [Validators.required]],
        categories: this.buildCategories(),
        tags: [null],
        imgFeatured: [null],
        slug: [null]
    });
}

buildCategories() {

    const values = this.categories.map(x => new FormControl(false));

    return this.formBuilder.group(values);
}

}

angular

1 answer

1

Squeal like that 

ngOnInit() {

this.categoriesService.read().subscribe(categories =>{ 
this.form = this.formBuilder.group({
    id: [null],
    title: [null, [Validators.required, Validators.minLength(3)]],
    body: [null, [Validators.required]],
    categories: this.buildCategories(categories ),
    tags: [null],
    imgFeatured: [null],
    slug: [null]
 });
});

buildCategories(categories) {

 const values = categories.map(x => new FormControl(false));

 return this.formBuilder.group(values);
}

This error is because it only receives the data in the request after calling the method

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