I want to build a javascript function that maintains state. Here's a pattern that I've come up with, but something in the back of my mind tells me this is an anti-pattern.
function f() {
var state = 1;
f = function() {
return state++;
};
return f();
};
Is there anything wrong with this? If so, what's a better approach?
Well it's a matter of opinion what the best way is, but (although I know it works) I'm a little uncomfortable with having the function overwrite itself. A similar pattern that doesn't do that but still uses practically the same closure idea is this:
var f = function() {
var state = 1;
return function() {
return state++;
};
}();
Or here is another way:
function f() {
return f.state++;
}
f.state = 1;
Of course with the f.state method the advantage and disadvantage (depending on your needs) is that the .state property can be read and modified by other code.
Normally, you set a closure scope and return a function that has access to that scope. Every time that function is now called, the state will remain as long as that function exists. Example:
var statefulFunction = function() {
// set up closure scope
var state = 1;
// return function with access to the closure scope
return function() {
return state++;
};
}(); // immediately execute to return function with access to closure scope
var first = statefulFunction(); // first === 1
var second = statefulFunction(); // second === 2
Another pattern is to create a closure scope and return an object with methods that have access to that closure scope. Example:
var myStatefulObj = function() {
// set up closure scope
var state = 1;
// return object with methods to manipulate closure scope
return {
incr: function() {
state++;
},
decr: function() {
state--;
},
get: function() {
return state;
}
};
}();
myStatefulObj.incr();
var currState = myStatefulObj.get(); // currState === 2
myStatefulObj.decr();
currState = myStatefulObj.get(); // currState === 1
myStatefulObj function to return the objectA better way to achieve this might be to use an Immediately-Invoked Function Expression (IIFE) to encapsulate your state.
var f = (function () {
var state = 1;
return function() {
return state++;
};
}());
console.log(f()); // 1
console.log(f()); // 2
f is, and then calling f().
; after state was a typo since JSLint has gotten me in the habit of declaring all of my variables comma delimited at the top of a function.