外部函数返回内部函数，可以从外部函数访问值

Question

I need to create a function which I will pass to my database server. The returned function will take a single item as a parameter and compare that item to a list of requirements.

For this I need a function generating function that takes an array as it's argument and returns the inner function which has that array built in to it.

Here's an example:

function create_query (list_of_requirements) {
  return function (item) {
    var is_match = true
    // the next function sets is_match to false if the item fails
    list_of_requirements.forEach(check_if_item_meets_requirement)
    return is_match
  }
}

An example of using this:

function search (parsed_user_string) {
  db.get(create_query(parsed_user_string)).then(function(results){
    show_results(results)
  })
}

How can I build the list of requirements into the inner function?

Answer 1

I needed to use a closure.

Here's a simpler example of a solution.

function makePrinter (num) {
  (return function () {
    print(num)
    })
}

and then:

var print5 = makePrinter(5)
print5() > prints 5!

I still don't quite get how closures accomplish this, but there it is.