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@let allows you to define a local variable and re-use it across the template.
Syntax 
angular-html
@let name = user.name;
@let data = data$ | async;Description 
@let declarations are similar to JavaScript's let and their values can be any valid Angular expression. The expressions will be re-evaluated everytime the template is executed.
Similarly to javascript variables, variables declared @let, can be used:
- After they have been declared
 - Within the bound of the same scope and nested scopes
 
angular-html
@let user = user$ | async;
@if (user) {
  <h1>Hello, {{user.name}}</h1>
  <user-avatar [photo]="user.photo"/>
  <ul>
    @for (snack of user.favoriteSnacks; track snack.id) {
      <li>{{snack.name}}</li>
    }
  </ul>
  <button (click)="update(user)">Update profile</button>
}Syntax definition 
The @let syntax is formally defined as:
- The 
@letkeyword. - Followed by one or more whitespaces, not including new lines.
 - Followed by a valid JavaScript name and zero or more whitespaces.
 - Followed by the = symbol and zero or more whitespaces.
 - Followed by an Angular expression which can be multi-line.
 - Terminated by the 
;symbol. 
HELPFUL: A comprehensive description of the feature is availble on the templates guide