关闭问题 - 什么是 JavaScript 语句来调用从封闭 function 返回的 function？

Question

This is my first time using a closure as seen below, but I want to try to call myFunction() from another place of code. What JS statement is required to invoke the anonymous function expression that is returned from the enclosing function labeled handleMouseDown() ? Currently handleMouseDown() is not being invoked on a mouseDown event on a select dropdown list that bind to the mouseDown.

function handleMouseDown(whichDropDown) {
            if (typeof mouseDownCount === 'undefined')
                mouseDownCount = 0;
            return function(whichDropDown, mouseDownCount) {
                switch (mouseDownCount) {
                    case 0:
                        mouseDownCount++;
                        break;
                    case 1:
                        mouseDownCount++;
                        myFunction(whichDropDown);
                        break;
                }
                return mouseDownCount;
            }
        }

Answer 1

Given your code, the only thing required for myFunction to be invoked looks to be the passing of 1 as an argument to the callback returned from handleMouseDown :

 const myFunction = arg => console.log('myFunction called with', arg); function handleMouseDown(whichDropDown) { if (typeof mouseDownCount === 'undefined') mouseDownCount = 0; return function(whichDropDown, mouseDownCount) { switch (mouseDownCount) { case 0: mouseDownCount++; break; case 1: mouseDownCount++; myFunction(whichDropDown); break; } return mouseDownCount; } } const callback = handleMouseDown('someDropdown'); callback('foo', 1);

Keep in mind that you appear to have two separate bindings for mouseDownCount - one looks to be an outer or global variable (not declared), and the other is the second argument passed to the callback. I'm not entirely sure what you're trying to implement there, but make sure you understand the difference, it'll probably be an easy source of bugs.

If you just want to call something on the second click of an element, it would be far easier just to have a variable that keeps track of the number of clicks:

 let clickCount = 0; window.addEventListener('click', () => { console.log('click detected'); clickCount++; if (clickCount === 2) { console.log('second click'); } });