Rules for the use of angle brackets in Typescript

Question

What are the general rules that dictate when, where, how and why angle brackets, i.e. \"<...>\", should be used in TypeScript?

Wh

Answer 1

With questions like this, I'd recommend reading the spec, especially the Grammar section. Syntax like < something > is used in

1. Type Parameters

   • Defined as < TypeParameterList > in section 3.6.1

   • Used with declarations and call signatures of classes, interfaces, functions and more

     function heat(food: T): T { return food; }
               //^^^^^ Type parameter list
     
     class Pizza { toppingA: T; toppingB: E }
              //^^^^^^^^^^^^^^^^^^^^ Type parameter list
     

2. Type Arguments

   • Defined as < TypeArgumentList > in section 3.6.2

   • Used with references to generic types and calls to generic functions

     var pizza: Pizza;
                    //^^^^^^^^^^^^^^^^^^^^^^ Type argument list
     pizza = heat<{ toppingA: Pepperoni, toppingB: Mozzarella}>(ingredients) 
                //^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Type argument list
     

     Update 2018-07-01: As of version 2.9, generic type arguments can also be used in JSX elements and tagged templates.

       toppings={[Pepperoni, Mozzarella]} />
             //^^^^^^^ Type argument list
     
      const ratingHtml = escapeUserInput `Customer ${feedback.customer.username} rated this pizza with ${feedback.rating}/10!`
                                       //^^^^^^^^^^^^^^^^ Type argument list
     

3. Type Assertions

   • Defined and used as < Type > UnaryExpression where UnaryExpression comes from EcmaScript standard in section 4.16

     var ingredients = {
         toppingA: new Pepperoni,
         toppingB:  fridge.takeSomeCheese()
                 //^^^^^^^^^^^^ Type assertion
     };
     

4. JSX expressions (when enabled)

   • Not documented in the spec, but should follow the the syntax of JSX, which is basically an expression like

      JSXChildren(optional) 
     

     or