You can't do this because the this context of getBear is not bound to an Animal when you call it. Simply telling TypeScript that this is an Animal isn't enough, you also have to call your function with that context.

In this case you would need to call it like this.

type Animal = {
    name: string
}

function getBear(this: Animal) : Animal {
    this.name = "hi"
    return this
}

console.log(getBear.call({ name: "test" }).name)

You can call this 3 ways:

type Animal = {
    name: string
}

function getBear(this: Animal, a: string): Animal {
    this.name = a
    return this
}

// First one 

// function.call(objcontext,parameters) calls a function under a different object context.
// read more at : https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/call

console.log(getBear.call({ name: "test" }, "Username").name)

// Here we create a object 
//second
console.log(new (getBear as any)("Username").name)

//third
console.log(new (<any>getBear)("Praveen").name)