why adding keyword `let` or `var` makes code uncompilable

Question

In following component class, if I add let keyword, the code doesn't compile. Why?

export class HelloComponent {
  @Input() name: string;
  name2:string;//let name2:string doesn't compile.

  constructor(){

  }
}

Answer 1

let and var are only required/allowed for local variables, not for class fields.

export class HelloComponent {
  @Input() name: string;
  name2:string;//let name2:string doesn't compile.

  constructor(){
    var x = 5; // ok
    let y = 5; // ok
    const z = 5; // ok
  }
}

In a class outside a method (or constructor) only variable initialization and method declaration is allowed, therefore let is redundant and therefore not allowed.