Typescript: Force Default Generic Type to be `any` instead of `{}`

Is it possible? (Without changing the function calls obviously. AKA without a().)

Yes.

I believe in your case you would do

var a = function<T = any>() : T  {
    return null;
}

Generic defaults were introduced in TS 2.3.

Default types for generic type parameters have the following syntax:

TypeParameter :
  BindingIdentifier Constraint? DefaultType?

DefaultType :
  `=` Type

For example:

class Generic<T = string> {
  private readonly list: T[] = []

  add(t: T) {
    this.list.push(t)
  }

  log() {
    console.log(this.list)
  }

}

const generic = new Generic()
generic.add('hello world') // Works
generic.add(4) // Error: Argument of type '4' is not assignable to parameter of type 'string'
generic.add({t: 33}) // Error: Argument of type '{ t: number; }' is not assignable to parameter of type 'string'
generic.log()

const genericAny = new Generic<any>()
// All of the following compile successfully
genericAny.add('hello world')
genericAny.add(4)
genericAny.add({t: 33})
genericAny.log()

See https://github.com/Microsoft/TypeScript/wiki/Roadmap#23-april-2017 and https://github.com/Microsoft/TypeScript/pull/13487

Is it possible? (Without changing the function calls obviously. AKA without a().)

No.

PS

Note that having a generic type that isn't actively used in any function parameters is almost always a programming error. This is because the following two are equavalent:

foo<any>() and <someEquvalentAssertion>foo() and leaves it completely at the mercy of the caller.

PS PS

There is an official issue requesting this feature : https://github.com/Microsoft/TypeScript/issues/2175

Taking this a little further for a slightly different application, but you can also default type arguments to another type argument.

For example, let's take:

class Foo<T1 = any, T2 = T1> {
    prop1: T1;
    prop2: T2;
}

and so:

const foo1 = new Foo();
typeof foo1.prop1 // any
typeof foo1.prop1 // any

const foo2 = new Foo<string>();
typeof foo2.prop1 // string
typeof foo2.prop2 // string

const foo3 = new Foo<string, number>();
typeof foo3.prop1 // string
typeof foo3.prop1 // number