Promise.prototype.finally

_index.md

Реализация Promise.prototype.finally
Имеет зависимость от SpeciesConstructor

finally.js

/// Практическая копия настоящего Promise.prototype.finally, выполнена в соответствии со спецификацией ECMAScript
Promise.prototype._finally = function(onFinally){
    const promise = this;
    if(typeof promise !== "object") throw TypeError("Promise.prototype._finally called on non-object");
    let thenFinally, catchFinally;
    const C = speciesConstructor(promise, Promise);
    if(typeof onFinally !== "function"){
      thenFinally = onFinally;
      catchFinally = onFinally;
    }else{
      thenFinally = (value) => {
        const result = onFinally();
        const promise = Promise.resolve.call(C, result);
        const valueThunk = () => value;
        return promise.then(valueThunk);
      }
      catchFinally = (reason) => {
        const result = onFinally();
        const promise = Promise.resolve.call(C, result);
        const throwReason = () => {throw reason};
        return promise.then(throwReason);
      }
    }
    return promise.then(thenFinally, catchFinally);
}