JavaScript Functions that Return Functions

A few weeks back, I tweeted that I loved functions that returned functions. I got quite a few replies to the tune of....WTF?!  It's important that people understand the value of functions that return functions;  using this technique can save you code, JavaScript efficiency, and a gained understanding of how powerful JavaScript can be.  I've created a quick example I'd like to show you so that you can get the idea I was trying to communicate.

Let's say you have one host object with two child objects, both with get methods, and both do exactly the same task but with a different attribute:

var accessors = {
	sortable: {
		get: function() {
			return typeof this.getAttribute('sortable') != 'undefined';
		}
	},
	droppable: {
		get: function() {
			return typeof this.getAttribute('droppable') != 'undefined';
		}
	}
};

Repeating the same code isn't ideal, so we could create one external function, passing it an attribute argument:

function getAttribute(attr) {
	return typeof this.getAttribute(attr) != 'undefined';
}
 
var accessors = {
	sortable: {
		get: function() {
			return getAttribute('sortable');
		}
	},
	droppable: {
		get: function() {
			return getAttribute('droppable');
		}
	}
};

That's a lot better but still not ideal because there's an extra, intermediate function execution every time the method is called.  What would work best is a function that returned the final function  -- that would eliminate the extra function execution with every call to get:

function generateGetMethod(attr) {
	return function() {
		return typeof this.getAttribute(attr) != 'undefined';
	};
}
 
var accessors = {
	sortable: {
		get: generateGetMethod('sortable')
	},
	droppable: {
		get: generateGetMethod('droppable')
	}
};

/* functional equivalent to the original code:

var accessors = {
	sortable: {
		get: function() {
			return typeof this.getAttribute('sortable') != 'undefined';
		}
	},
	droppable: {
		get: function() {
			return typeof this.getAttribute('droppable') != 'undefined';
		}
	}
};

*/

What you see above is a function returning a function; each method gets its own method for getting the property and there's no overhead upon each get call.

This is a really useful technique that saves you from repeating likewise code and, when used correctly, is easy to understand and maintain!

原文链接：https://davidwalsh.name/javascript-functions