Return to Answer

This is an attempt to clear up several (possible) misunderstandings about closures that appear in some of the other answers.

• A closure is not only created when you return an inner function. In fact, the enclosing function does not need to return at all in order for its closure to be created. You might instead assign your inner function to a variable in an outer scope, or pass it as an argument to another function where it could be called immediately or any time later. Therefore, the closure of the enclosing function is probably created as soon as the enclosing function is called since any inner function has access to that closure whenever the inner function is called, before or after the enclosing function returns.
• A closure does not reference a copy of the old values of variables in its scope. The variables themselves are part of the closure, and so the value seen when accessing one of those variables is the latest value at the time it is accessed. This is why inner functions created inside of loops can be tricky, since each one has access to the same outer variables rather than grabbing a copy of the variables at the time the function is created or called.
• The "variables" in a closure include any named functions declared within the function. They also include arguments of the function. A closure also has access to its containing closure's variables, all the way up to the global scope.
• Closures use memory, but they don't cause memory leaks since JavaScript by itself cleans up its own circular structures that are not referenced. IE memory leaks involving closures are created when it fails to disconnect DOM attribute values that reference closures, thus maintaining references to possibly circular structures.

This is an attempt to clear up several (possible) misunderstandings about closures that appear in some of the other answers.

• A closure is not only created when you return an inner function. In fact, the enclosing function does not need to return at all in order for its closure to be created. You might instead assign your inner function to a variable in an outer scope, or pass it as an argument to another function where it could be called immediately or any time later. Therefore, the closure of the enclosing function is probably created as soon as the enclosing function is called since any inner function has access to that closure whenever the inner function is called, before or after the enclosing function returns.
• A closure does not reference a copy of the old values of variables in its scope. The variables themselves are part of the closure, and so the value seen when accessing one of those variables is the latest value at the time it is accessed. This is why inner functions created inside of loops can be tricky, since each one has access to the same outer variables rather than grabbing a copy of the variables at the time the function is created or called.
• The "variables" in a closure include any named functions declared within the function. They also include arguments of the function. A closure also has access to its containing closure's variables, all the way up to the global scope.
• Closures use memory, but they don't cause memory leaks since JavaScript by itself cleans up its own circular structures that are not referenced. Internet Explorer memory leaks involving closures are created when it fails to disconnect DOM attribute values that reference closures, thus maintaining references to possibly circular structures.

This is an attempt to clear up several (possible) misunderstandings about closures that appear in some of the other answers.

• A closure is not only created when you return an inner function. In fact, the enclosing function does not need to return at all in order for its closure to be created. You might instead assign your inner function to a variable in an outer scope, or pass it as an argument to another function where it could be called immediately or any time later. Therefore, the closure of the enclosing function is probably created as soon as the enclosing function is called since any inner function has access to that closure whenever the inner function is called, before or after the enclosing function returns.
• A closure does not reference a copy of the old values of variables in its scope. The variables themselves are part of the closure, and so the value seen when accessing one of those variables is the latest value at the time it is accessed. This is why inner functions created inside of loops can be tricky, since each one has access the same outer variables rather than grabbing a copy of the variables at the time it is called.
• The "variables" in a closure include named functions declared within the function. It also includes function arguments. A closure also has access to its containing closure's variables, all the way up to the global scope.
• Closures use memory, but they don't cause memory leaks since JavaScript by itself cleans up its own circular structures that are not referenced. IE memory leaks involving closures are created when it fails to disconnect DOM attribute values that reference closures, thus maintaining references to possibly circular structures.

This is an attempt to clear up several (possible) misunderstandings about closures that appear in some of the other answers.

• A closure is not only created when you return an inner function. In fact, the enclosing function does not need to return at all in order for its closure to be created. You might instead assign your inner function to a variable in an outer scope, or pass it as an argument to another function where it could be called immediately or any time later. Therefore, the closure of the enclosing function is probably created as soon as the enclosing function is called since any inner function has access to that closure whenever the inner function is called, before or after the enclosing function returns.
• A closure does not reference a copy of the old values of variables in its scope. The variables themselves are part of the closure, and so the value seen when accessing one of those variables is the latest value at the time it is accessed. This is why inner functions created inside of loops can be tricky, since each one has access to the same outer variables rather than grabbing a copy of the variables at the time the function is created or called.
• The "variables" in a closure include any named functions declared within the function. They also include arguments of the function. A closure also has access to its containing closure's variables, all the way up to the global scope.
• Closures use memory, but they don't cause memory leaks since JavaScript by itself cleans up its own circular structures that are not referenced. IE memory leaks involving closures are created when it fails to disconnect DOM attribute values that reference closures, thus maintaining references to possibly circular structures.

This is an attempt to clear up several (possible) misunderstandings about closures that appear in some of the other answers.

• A closure is not only created when you return an inner function. In fact, the enclosing function does not need to return at all. You might instead assign your inner function to a variable in an outer scope, or pass it as an argument to another function where it could be used immediately. Therefore, the closure of the enclosing function probably already exists at the time that enclosing function was called since any inner function has access to it as soon as it is called.
• A closure does not reference a copy of the old values of variables in its scope. The variables themselves are part of the closure, and so the value seen by accessing one of those variables is the latest value at the time it is accessed. This is why inner functions inside of loops can be tricky, since they all access the same outer variables.
• The "variables" in a closure include named functions declared (at the top level) within the function. It also includes function arguments. The closure also has access to its containing closure's variables, all the way up to the global scope.
• Closures use memory, but they don't cause memory leaks since JavaScript itself cleans up circular structures that have no references. IE memory leaks involving closures are caused by not disconnecting DOM attribute values that reference closures, thus maintaining references to possibly circular structures.

This is an attempt to clear up several (possible) misunderstandings about closures that appear in some of the other answers.

• A closure is not only created when you return an inner function. In fact, the enclosing function does not need to return at all in order for its closure to be created. You might instead assign your inner function to a variable in an outer scope, or pass it as an argument to another function where it could be called immediately or any time later. Therefore, the closure of the enclosing function is probably created as soon as the enclosing function is called since any inner function has access to that closure whenever the inner function is called, before or after the enclosing function returns.
• A closure does not reference a copy of the old values of variables in its scope. The variables themselves are part of the closure, and so the value seen when accessing one of those variables is the latest value at the time it is accessed. This is why inner functions created inside of loops can be tricky, since each one has access the same outer variables rather than grabbing a copy of the variables at the time it is called.
• The "variables" in a closure include named functions declared within the function. It also includes function arguments. A closure also has access to its containing closure's variables, all the way up to the global scope.
• Closures use memory, but they don't cause memory leaks since JavaScript by itself cleans up its own circular structures that are not referenced. IE memory leaks involving closures are created when it fails to disconnect DOM attribute values that reference closures, thus maintaining references to possibly circular structures.