Objects

A Javascript object has a lot in common with a C# dictionary.

You could imagine it as a dynamic type in C# that looks up values in a dictionary.

Methods are object properties that reference a function.

this is the calling context, not the current class.

Functions

Can return a reference to themselves.

There is an arguments implicit parameter.

All parameters are optional.

Unspecified parameters are set to undefined

Mixins

Null coalescing

CSharp

    var value = GetPossiblyNullValue() ?? "Default value";

Javascript

var value = getPossiblyNullValue() || "Default value";

Globals

Anything not defined within a function or using the var keyword, will be set on the global window object.

Arrays

Array is more like Dictionary<int, object> where the index is an auto-incrementing value.

Classes

No class in javascript, but there is a new keyword.

Prototypical inheritance etc;

Inheritance polyfill code:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/create

if (typeof Object.create != 'function') {
  Object.create = (function() {
    var Temp = function() {};
    return function (prototype) {
      if (arguments.length > 1) {
        throw Error('Second argument not supported');
      }
      if (typeof prototype != 'object') {
        throw TypeError('Argument must be an object');
      }
      Temp.prototype = prototype;
      var result = new Temp();
      Temp.prototype = null;
      return result;
    };
  })();
}

Closures

Polymorphism

Functions cannot be overloaded in Javascript

Equality & Truthiness

In C#, you have null as your default value for empty references.

In JavaScript, you have null, must also undefined.

In C#, conditional tests must evaluate to a boolean value or they won’t even compile.

e.g. this is illegal in C#:

var value = "Non-null string value";
if(value) Console.WriteLine("Say something");

Equals

In JavaScript, == is a soft comparison. You can use === for hard comparison, which strictly matches type and value, and is more equivalent to .Equals or == in C#.

In JavaScript, any value that is null, 0, empty or undefined is considered false.

"" == false;
0 == false;
null == false;

Objects are only equal to themselves.

"yes" == "yes";
"yes" === "yes";
1 == -1;
1 === -1;

Multi-line strings

C#:

var value = @"First line
Second line
Third line";

JavaScript:

Use backticks:

var value = `First line
Second line
Third line`;

Anonymous functions

Lambdas

CSharp

var expression = () => "Test";

Javascript

var expression = () => "Test";

Promises

Javascript is single-threaded, meaning blocking and long-running calls can make websites unusable.

Node.js is also single-threaded, but can scale massively by using non-blocking, asynchronous programming.

The traditional JavaScript asynchronous programming model involves specifying callback functions to blocking or long-running code.

Promises seek to standardize this pattern, and provide native support for promises in modern browsers.

It allows easily chaining asynchronous calls in a readable, consistent manner, while reducing nesting and simplifying error handling.

Asynchronous programming came to .NET as a first class citizen with async and await on top of the existing Task libraries.

Promises can be changed; if you return a value, that gets passed to the next then. If you return a “thenable”, then it will get resolved before the return value getting passed to the next then.

Development tips and tricks

Debugging

You can put the debugger; statement anywhere to force a breakpoint in development tools.

In all modern browsers, you can hit F12 to open the developer tools