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If a variable is declared in a higher scope, it can be accessed by child scopes. When JavaScript tries to resolve an identifier, it looks in the local scope. If this identifier is not found, it looks in the next outer scope, and so on along the scope chain until it reaches the global scope where global variables reside.
A variable can be either of global or local scope. A global variable is a variable declared in the main body of the source code, outside all functions, while a local variable is one declared within the body of a function or a block. Modern versions allow nested lexical scope.
The key to understanding design patterns such as IIFE is to realize that prior to ES6, JavaScript only featured function scope (thus lacking block scope), passing values by reference inside closures. [9] This is no longer the case, as the ES6 version of JavaScript implements block scoping using the new let and const keywords. [10]
Local variables may have a lexical or dynamic scope, though lexical (static) scoping is far more common.In lexical scoping (or lexical scope; also called static scoping or static scope), if a variable name's scope is a certain block, then its scope is the program text of the block definition: within that block's text, the variable name exists, and is bound to the variable's value, but outside ...
Interaction mechanisms with global variables are called global environment (see also global state) mechanisms. The global environment paradigm is contrasted with the local environment paradigm, where all variables are local with no shared memory (and therefore all interactions can be reconducted to message passing ).
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The term closure is often used as a synonym for anonymous function, though strictly, an anonymous function is a function literal without a name, while a closure is an instance of a function, a value, whose non-local variables have been bound either to values or to storage locations (depending on the language; see the lexical environment section below).
In computer programming, variable shadowing occurs when a variable declared within a certain scope (decision block, method, or inner class) has the same name as a variable declared in an outer scope. At the level of identifiers (names, rather than variables), this is known as name masking .