Saturday, July 23, 2016

You don't know Javascript (Scope and Closure)

Javascript is one of the 3 main core technologies in world wide web. Since it's origin 2 decades back, it has evolved a lot and as per today, it is the trendiest programming language on earth.

Since 2009 I have used javascript to do web development and until recently I used it with basic dom manipulations with jquery, simple UI logic with vanilla JS and with some frameworks as AnuglarJS. The basic knowledge I had about JS was sufficient enough to get above things done. But recently when I started doing some serious development with React and GraphQL, I realized that my knowledge on JS is pretty primitive and I'm not really applying any core JS concepts or patterns in my code. At the same time, while I was listing to a podcast in Software Engineering Daily I found Kyle Simpson was explaining about series of his books called "You Don't know Javascript". With that, I got the interest in reading the book to sharpen my saw in JS. It's a pretty interesting book and it covers many basic concepts in JS. Following series of articles will cover what I learned in each of 
these books.

Scope


The scope is one of the core concepts in Javascript and it's a must to have a thorough understanding of the scope to write better JS code. Let's dig in further to scope.

What is scope?

The scope is set of well-defined rules on how to store and retrieve variables in a program. In one way or other, you are always dealing with these rules when writing and execute code in JS. 


In JS assignment for the scope can be performed in two ways.

1) by using = expression

a = 3;

2) or by passing an argument to a function parameter.

function foo(x) {
}

foo(3);

Lexical Scope

In JS, the scope is defined by Lexical Scope. Lexical Scope is defining the scope at author time based on where the functions are placed.

But there are two other ways to overwrite the lexical scope in JS. Those are JS core functions which will manipulate the scope at runtime. Those are,

1) eval() function
Eval function takes a string as an input and executes the content in the scope of the code execution.

2) with() function
Takes an object as the input and consider the object as a new separate lexical scope.


function foo(obj) {
  with(obj){
    console.log(x);
  }
}
foo({x:5}); // will print "5"

Cheating lexical scope makes the code performance intensive so better to avoid it.


Nested Scopes

In JS we can define different levels of scopes. The most outer scope is global scope and new scopes will be defined as nested scopes when we are creating functions or blocks.

var a = 5;

function foo(x) {
  var b = 10;
}
In above example "a" is in global scope and "b" is in a new function scope.

When Javascript engine finds a variable in an inner scope engine starts searching for it in the given scope and if it isn't there, it checks the variable in the immediate outer scope and likewise it traverse until it finds the variable and returns it. Once it reaches the most outer scope, it automatically stops traversing.

There is a really tricky part in how the fall back happens when the engine cannot find a defined variable in any given scope. If a variable is not found in any parent scope and if it's for an assignment purpose, then a new variable will be created in the global scope. You should be really careful on this.

var a = 5;

function foo(x) {
  b = 10;
}

When Javascript engine finds a variable in an inner scope engine starts searching for it in the given scope and if it isn't there, it checks the variable in the immediate outer scope and likewise it traverse until it finds the variable and returns it. Once it reaches the most outer scope, it automatically stops traversing.

Note: Global variables are automatically being part of the global object. In browser, it's the window object. So you can create global variables as "window.a". Using this technique you can access the variables in the global scope without getting affected from shadowing. Ex:

var a = 5;

function foo() {
  var a = 10;
  console.log(a); // will print "10"
  console.log(window.a); // will print "5"
}

foo()

This will print two lines as follows.

10
5

There is another interesting point demonstrated in the above example. It's shadowing.

Shadowing

Overriding the outer scope variables in inner scopes. Shadowing is commonly used in JS which allows developers to isolate variables within functions while using same variable names.

Since scope is really powerful as well as dangerous when misusing it, there are identified best practices to avoid surprises. All these best practices are targeting common objectives such as avoiding spoiling global scope, isolate variables in function scope as much as possible. Following are some of such best practices.

Isolate in objects

It's a best practice to create an object in the global namespace and define variables and functions as properties of the object. In this way, we will only define a limited number of variables in the global scope and all the other variables will be properties of carrier objects.

var myLibrary = {
  foo: function() {
    console.log('this is foo');
  },
  x: 5
}

myLibrary.foo(); // will print "this is foo"


Module Managment

Most of the modern JS libraries wraps its variables in a module and expose via dependency management. When using these libraries in code, we have to import scope into another object scope which will prevent spoiling the global scope. 

Will be covered more under Closure section.

Functions as Scope

When we have named functions, it pollutes the global name space. That can be avoided with following technique by treating function as an expression rather than a declaration.


(function foo() {
    console.log('How are you');
})(); // will print "How are you"


Also you can pass a variable into the function as follows.

(function foo(x) {
   console.log(x);
})("Hi"); // will print "Hi"


In above code first () make the function an expression and second () execute the expression.

Anonymous vs Named functions

Function expression can be anonymous but function declaration cannot be anonymous. It's prefer to have named function which has less drawbacks than anonymous functions. 

Following are valid syntaxes because there the function is an expression.

var x = function(x) {
   console.log(x);
}

x('wow'); // will print "wow"


(function(x) {
   console.log(x);
}
)('wow'); // will print "wow"

Following syntax is invalid because the there we use a function declaration.

function(x) {
   console.log(x);
}


Blocks as Scope

Generally if we define a variable with var keyword it belongs to the scope beyond the block and applicable for the whole scope.


if (true) {
  var n = "How are you";
}

console.log(n); // will print "How are you"


In above code variable "n" will belong to global scope. That happens becuase of variable hoisting in JS engine.

Variable Hoisting

Variables and function declarations are move to the top of the scope block. In here subsequent function declarations will override the previous.



a = 2;
var a;
console.log(a) // will print 2

console.log(a); // will print undefined
a = 2;
var a;

But if you define a variable with "let" keyword, it'll only belong to the current block. Variable declarations with "let" keyword will not effect to variable hoisting.


if (true) {
  let m = "How are you";
}

console.log(m); // will give an error

You can create a constant variable using const keyword and it's scope will be limited to the current block.


if (true) {
  const m = "How are you";
  m = "Changed"; // will give an error

}

Closure



What is Closure?

Closure is when a function is able to remember and use it's lexical scope even when the function is executing outside of the lexical scope.

Closure is one of the least understood features  in JS, but it is one of the most powerful features in JS. 

function foo(a)
  function bar() {
    console.log(a);
  }
  return bar;
}

var baz = foo(2);
baz() // will print 2


In above example, "bar" still has the reference to the scope of a function, and that reference is called closure.

Closure is heavily used in JS programming all over the programs. One of it's main application is there when developers create modules to isolate and expose variables and functions to outside.

Modules

Function return an object with references for internal functions. Those function will run outside the defined lexical scope and it's a good example for a closure.



function myModule(a, b) {
  function sum() {
    return a +b;
  }

  function substract() {
    return a - b;
  }

  return {
    sum: sum,
    substract: substract
  }
}

var operations = myModule(2, 4);
operation.sum() // Will print 6

This is a great way to keep the global scope clean as well expose necessary components to outside while maintaining an internal state of the module.


Now we have come to the end of this article about Javascript Scope and Closure concepts. Hope you got some quick interesting points about them and if you need further readings it's highly recommend to read the series of  "You Don't know Javascript" books. I'll bring some more interesting points in my next article about Javascript Objects and Prototyping. 

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