So, JavaScript is really a simple language. It is a language that is straight forward: what you see is what you get. Here is the problem: Closures are being used all over the place in modern JavaScript, but just because I see them does not mean I get them. This “simple” onClick and mouseOver language has got some sting.
This article is going to introduce a concept which (in my opinion) can be difficult to digest by itself, but is a fundamental (if not THE fundamental) building block to understanding the syntax that is very often used in “modern” JavaScript pages.
Obligatory simplified example:
<script language="JavaScript"> var validateForm=function(){ var checkValue = function(pattern,value){ } //The following calls work. They can "see" the checkValue function if (checkValue(//,document.getElementByID(''.) && checkValue(//,form.){ return true; } else { return false } } // checkValue(//,'abc'); // This fails. – can't "see" the function </script>
The checkValue variable is available only within the outer validateForm function. The checkValue is a closure. A function defined within another function is a closure. Functions that are defined within another function can “see” the variables within that function – var variables, arguments, and global variables.
Simple. A closure is a function defined within another function. That is pretty simple. The “what” is the easy part. The harder part is the “why” — and, even harder, all the nuances that they enable.
There are many code designs that we can use because of closures. We will start with the building blocks and over the next few articles, show some fantastic applications of this pattern to achieve some mind-blowing code.
First back to var. Free Choice.
Remember the var keyword? The var keyword scopes a variable to the function within which it is defined.
In a prior article, we played with var and saw the dangers of scoping variables in the global namespace. Here it is again: A closure is not automatically in the global namespace. In this example, the checkValue is a variable scoped to the outer validateForm function. In the above example, the only global variable is the validateForm variable.
Chew on this. What would removing the var in front of checkValue mean?
var validateForm=function(){ # NO var keyword below checkValue = function(pattern,value){ } //The following calls work. They can "see" the checkValue function if (checkValue(//,document.getElementByID(''.) && checkValue(//,form.){ return true; } else { return false } }
Answer: Removing the var has made that function visible everywhere. We get to choose. We can choose to add a variable to either function scope of global scope. I like choice.
Second: More var. Private variables
<script language="JavaScript"> var createMessage = function(msg){ var privateMessage = msg var show = function(){ console.log(privateMessage); } return {'show':show} } var happyMessage = createMessage('Hello Friends'); var sadMessage = createMessage('Goodbye Friends'); happyMessage.show(); sadMessage.show(); // This is no good. This will not work. //console.log(privateMessage); </script>
The privateMessage variable is created as a var variable within the createMessage function. It is not accessible outside that function. The closure (show) is making a reference to that variable. We can see variables from the outer function inside a closure. In fact, it gets better. That state – the state of that variable – stays around as long as we hold reference to the show variable. We are passing the show variable back out. That show function is able to be invoked from outside the createMessage function. The specific state of the privateMmessage variable is retained on the “stack” so the show function has access to it.
Did you catch that? We created a private variable that is only accessible from the show function. Do you suppose we can modify that private value? Check it out:
<script language="JavaScript"> var createMessage = function(msg){ var privatemessage = msg var show = function(){ console.log(privatemessage); } var setValue = function(msg){ privatemessage = msg; } return {'show':show,'setValue':setValue} } var happyMessage = createMessage('Hello Friends'); var sadMessage = createMessage('Goodbye Friends'); happyMessage.show(); sadMessage.show(); happyMessage.setValue('Wahoo Glad you are still here!'); happyMessage.show(); sadMessage.show(); </script>
Don’t look now, but I believe this is an implementation of encapsulation; Variable encapsulation in JavaScript using closures.
There are MANY more nuggets that this closure concept enables. Keep an eye out for a few more articles that will dive much deeper into the “Why” and patterns around this.
Author: Todd Wright, one of Accelebrate’s instructors
Accelebrate offers private JavaScript training for groups and instructor-led online JavaScript classes for individuals.
Written by Todd Wright
Our live, instructor-led lectures are far more effective than pre-recorded classes
If your team is not 100% satisfied with your training, we do what's necessary to make it right
Whether you are at home or in the office, we make learning interactive and engaging
We accept check, ACH/EFT, major credit cards, and most purchase orders
Alabama
Birmingham
Huntsville
Montgomery
Alaska
Anchorage
Arizona
Phoenix
Tucson
Arkansas
Fayetteville
Little Rock
California
Los Angeles
Oakland
Orange County
Sacramento
San Diego
San Francisco
San Jose
Colorado
Boulder
Colorado Springs
Denver
Connecticut
Hartford
DC
Washington
Florida
Fort Lauderdale
Jacksonville
Miami
Orlando
Tampa
Georgia
Atlanta
Augusta
Savannah
Hawaii
Honolulu
Idaho
Boise
Illinois
Chicago
Indiana
Indianapolis
Iowa
Cedar Rapids
Des Moines
Kansas
Wichita
Kentucky
Lexington
Louisville
Louisiana
New Orleans
Maine
Portland
Maryland
Annapolis
Baltimore
Frederick
Hagerstown
Massachusetts
Boston
Cambridge
Springfield
Michigan
Ann Arbor
Detroit
Grand Rapids
Minnesota
Minneapolis
Saint Paul
Mississippi
Jackson
Missouri
Kansas City
St. Louis
Nebraska
Lincoln
Omaha
Nevada
Las Vegas
Reno
New Jersey
Princeton
New Mexico
Albuquerque
New York
Albany
Buffalo
New York City
White Plains
North Carolina
Charlotte
Durham
Raleigh
Ohio
Akron
Canton
Cincinnati
Cleveland
Columbus
Dayton
Oklahoma
Oklahoma City
Tulsa
Oregon
Portland
Pennsylvania
Philadelphia
Pittsburgh
Rhode Island
Providence
South Carolina
Charleston
Columbia
Greenville
Tennessee
Knoxville
Memphis
Nashville
Texas
Austin
Dallas
El Paso
Houston
San Antonio
Utah
Salt Lake City
Virginia
Alexandria
Arlington
Norfolk
Richmond
Washington
Seattle
Tacoma
West Virginia
Charleston
Wisconsin
Madison
Milwaukee
Alberta
Calgary
Edmonton
British Columbia
Vancouver
Manitoba
Winnipeg
Nova Scotia
Halifax
Ontario
Ottawa
Toronto
Quebec
Montreal
Puerto Rico
San Juan