Tag: theory

bookmark_borderWhat is hoisting in JavaScript?

Posted on by kalkus

What is hoisting is one of the standard JavaScript recruitment questions. For some devs it’s one of the quirks of JavaScript, for others it allows understanding the language deeper. Let’s take a look at the example:

console.log(foo);  //returns undefined not ReferenceError exception 
var foo;

Console.log will not throw the exception. It will print undefined.

Why?

In most of the programming languages, we should have some ‘variable undeclared’ exception. In JavaScript, when the code is executed, the declarations of the variables (declared using var keyword, but not let!) and functions are hoisted, moved to the top (of the scope).

Important thing to notice is that initializations are not hoisted.

console.log(foo);  //returns undefined  
var foo = “bar”;

But why do we need it?

Well, if we need it or not is a good question, but let’s take a look at the example of how hoisting can be useful. As I said before – declarations of functions are also moved up. Therefore we can use the function before it’s declared:

sayHello("Mark");

function sayHello(name) {
  console.log("Hello " + name);
}

To read more about the hoisting see: https://developer.mozilla.org/en-US/docs/Glossary/Hoisting

bookmark_borderWhat does question mark and dot operator ?. mean in C# 6.0?

Posted on by kalkus

C# 6.0 introduced new operator – null conditional operator ?.

The way it works is very simple. It checks if the left part of the expression is null and if it’s not it returns the right part. Otherwise, it returns null.

Simple example:

var thisWillBeNulll = someObject.NullProperty?.NestedProperty;
var thisWilllBeNestedProperty = someObject.NotNullProperty?.NestedProperty;

Executable example:

//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Person
    {
        public string Name { get; set; }
    }
	
    public class Dog 
    {
        public Person Owner { get; set; }
    } 
    
    public class Program
    {
        public static void Main(string[] args)
        {
            Dog dog = new Dog();
            Console.WriteLine(dog.Owner?.Name == null);
            // this will print True
            dog.Owner = new Person() { Name = "Fred" };
            Console.WriteLine(dog.Owner?.Name); 
            // this will print Fred
        }
    }
}

The above example can be executed here: https://rextester.com/TBC19437

Documentation: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/operators/member-access-operators#null-conditional-operators–and-