The enigma of TypeScript

Whenever I try to declare or initialize data members in a class, the following methods never seem to work:

var view: string[];
var view: string[] = [];
let view: string[];
let view: string[] = [];

Even though the TypeScript documentation states that it should work as expected.

Click here for TypeScript basic types documentation Or visit this link for the TypeScript tutorial

However, when I use the following code snippet:

view: string[];

constructor() {
    this.view = [];
}

Everything works fine without any issues.

Could someone please explain why the following code results in an error:

export class TextComponent implements OnInit {

   let view: string[] = [];        

    ngOnInit() {
    ....
    }
}

And why using any of the previously mentioned variants always leads to the error:

ngOnInit is not defined

Answer №1

It's important to note that you can't place random code at the top level of a class in TypeScript. You must either include it within the constructor or a method.

Outside of constructors and methods, only field and method declarations are permitted.

export class TextComponent implements OnInit {

    view: string[] = [];        

    constructor() {
      var view: string[];
      var view: string[] = [];
      let view: string[];
      let view: string[] = [];
    }    
    ngOnInit() {
    ....
    }
}

I have not tested whether declaring the same variable multiple times inside the constructor would cause any issues, but as long as you follow this structure, it should function correctly.

Answer №2

Your code snippet:

export class TextComponent implements OnInit {

   let view: string[] = [];  

An error occurred during compilation (invalid syntax)!. Make sure to check the console for more information.

Solution

Ensure you are using the correct syntax for initializing class members.

export class TextComponent implements OnInit {

   view: string[] = [];  

Additional Information

https://basarat.gitbooks.io/typescript/content/docs/classes.html

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