JS closure knowledge

One ： The three elements of closures ：
1. Functions are nested
2. Internal functions must reference local variables of external functions
3. External functions must call
Two ： Closure advantages ：
1. Can be outside the function Access to internal variables （ Originally, local variables inside the function cannot be accessed outside the function , But the function inside the function can access the local variables in this function ）
2. Extend the life cycle of local variables （ The essence ：）
var fnn2 = fn() The internal variable was originally executed once a It should be destroyed , But because it is assigned a value, there is a reference relationship , Local variables are always saved
3、 ... and ： Disadvantages of closures
1. Abusing closures , Its execution environment has always been Cause memory overflow And memory leaks
Four ： The life cycle of closure
1. produce ： Closures complete nested functions （ Three elements of establishment ） It occurs when ( Instead of calling )
2. Death ： When a variable that receives a closure is called a garbage object （null）
5、 ... and ： Solutions to closure shortcomings
1. Memory leak ： Memory could not be released
2. out of memory ： Memory burst
3. solve ：f=null ; Let the closure mechanism clear ,f Refers to the variable that receives the external function
6、 ... and ： Closure scope ： Start looking up where the function is created

1. Function is returned as a return value

2. The function is passed as an argument

7、 ... and ： Application scenarios of closures
1. Do the cache

2. throttle , Shake proof ： Limit the number of executions of the function
3. throttle throttle： Reduce the trigger frequency for a period of time （ Specify how long it can be triggered ）
① Timer version

② Timestamp version ：

4. Shake proof debounce： adopt setTimeout The way , Within a time interval of , Turn multiple triggers into one trigger , Only for the last time
Be careful this Points and parameters arguments problem