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Chapter 7. Scope and Closures

Conceptual Overview of JavaScript Scope

In JavaScript, scope is the context in which code is executed, and there are three types of scope: global scope, local scope (sometimes referred to as “function scope”), and eval scope.

Code defined using var inside of a function is locally scoped, and is only “visible” to other expressions in that function, which includes code inside any nested/child functions. Variables defined in the global scope can be accessed from anywhere because it is the highest level/last stop in the scope chain.

Examine the code below and make sure you understand that each declaration of foo is unique because of scope.

Live Code

<!DOCTYPE html><html lang="en"><body><script>

var foo = 0; // global scope
console.log(foo); // logs 0

var myFunction = function() {

   var foo = 1; // local scope

   console.log(foo); // logs 1

   var myNestedFunction = function() {

       var foo = 2; // local scope

       console.log(foo); // logs 2
   }();
}();

eval('var foo = 3; console.log(foo);'); // eval() scope

</script></body></html>

Please notice that each foo variable contains a different value because each one is defined in a specifically delineated scope.

Notes

  • An unlimited number of function and eval scopes can be created, while only one global scope is used by a JavaScript environment.

  • The global scope is the last stop in the scope chain.

  • Functions that contain functions create stacked execution scopes. These stacks which are chained together are often referred to as the scope ...

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