Javascript scoping

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Javascript scoping

  1. 1. Scoping and functions inJavascript Aditya Gaur Mentor: Apurba Nath
  2. 2. Why Scope? Controls the visibility of variables and parameters A programmer should know about scope to prevent naming collisions and ambiguity
  3. 3. Why Scope in Javascript? Javascript is NOT “s ome othe r la ngua ge ” Unlike other languages like C, C++, Javascript does not support bloc k le ve l s c ope Supports function level scope
  4. 4. Block level scope ? var a =10 ; Output 20 if(true){ var a = 20; } alert(a); var a =10 ; Output 10 function randomFunc(){ var a = 20; } randomFunc(); alert(a);
  5. 5. Declaration and Definition var someVar = 10; is broken into var someVar=undefined; someVar =10;
  6. 6. Declaration and Definition function someFunc(){ //some random statements var aVar = 10; //some more random statements } is converted to function someFunc(){ var aVar = undefined //some random statements aVar = 10; //some more random statements }
  7. 7. The var function Dummy(){ Output: Error- privateProperty is not var privateProperty = 10; defined } alert (privateProperty); function Dummy(){ Output: 10 isItPrivate = 10; } alert(isItPrivate);So whats happening? Actually the keyword var defines the scope of the variable. Ifwe dont give the keyword var then it is assigned to the window object
  8. 8. The eval functioneval("var a=10"); Output: 10alert(a);function someFunc(){ Output: 10 eval("var a=10"); a is not defined alert(a);}someFunc();alert(a);Variables declared in the eval have the scope in which they arecalled.
  9. 9. this is tricky It is one of the most important concept to grasp in javascript Its value depends in the way the function has been called. A function can be called in the following ways: – As a method – As function invocation – As a constructor – In apply invocation
  10. 10. 1. Function as a methodvar aVar=10; Output: 40var obj= { aVar: 40, show: function(){ alert(this.aVar); }};obj.show();this is bound to the object from which the method isinvoked.
  11. 11. 2. function invocationvar aVar=10; Output: 10function someFunc(){ var aVar =40; alert(this.aVar);}someFunc();Here the value of this is bound to the global object.
  12. 12. 3. function as a constructor function Dummy(){ Output: undefined var privateProperty = 10; } var dummyObject = new Dummy(); alert(dummyObject.privateProperty);function Dummy(){ Output: 10 this.publicProperty = 10;}var dummyObject = new Dummy();alert(dummyObject.publicProperty);this is bound to the new object being created so it is accessible through theobject.
  13. 13. Privileged method function Dummy() { var privateProperty = 10; this.privilegedMethod = function() { return privateProperty; }; } var dummyObject = new Dummy(); alert(dummyObject.privilegedMethod());
  14. 14. 4. function in apply invocation var obj = { x: 15 }; function dummy(message) { alert(message); alert(this.x); } dummy.apply(obj, ["invoking dummy through apply"]); Output: invoking dummy through apply 15Here the value of this is bound to the scope passed as the first argument in theapply function
  15. 15. A catchfunction Dummy(){ return function (){ alert (this); }();}var dummyObject = new Dummy();Here this refers to the global object. Why?The anonymous function here is executed immediately thus it is just like thefunction invocation and in function invocation this refers to the global windowobject
  16. 16. A fix function Dummy() { var that = this; return function (){ alert (that); }(); } var dummyObject = new Dummy();Now we have the correct behaviour. This is possible because javascriptsupports function closure.
  17. 17. Current and Future Work Working on – JSDocs – for the documentation – Integrated JSdocs in the build of the mVisualizer For unit testing would work either on – JSUnit – YUI test
  18. 18. Current and Future Work
  19. 19. References Javascript: the good parts by Douglas Crockford YUI theater videos by Douglas Crockford
  20. 20. Thank You

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