Develop Reference

Javascript: How to get key of function.apply()

I am trying to cache 'func.apply(this, func)' value so that it can be looked up later rather than running the function again. The problem is that I don't know how or what to use as the key.
Is there a way to assign an key of a function that can be looked up later?
Code example:
var m = function(func) {
var cached = {};
return function() {
var key = ''; // how do I get or create the key of func.apply(this, func)?
if (cached[key]) {
return cached[key];
}
cached[key] = func.apply(this, arguments);
return cached[key];
};
};
The m() function should return a function that, when called, will check if it has already computed the result for the given argument and return that value instead if possible.

What are you looking for is called Memoization
See: Implementing Memoization in JavaScript
Here are an example:
var myFunction = (function() {
'use strict';
var functionMemoized = function() {
// set the argumensts list as a json key
var cacheKey = JSON.stringify(Array.prototype.slice.call(arguments));
var result;
// checks whether the property was cached previously
// also: if (!(cacheKey in functionMemoized.cache))
if (!functionMemoized.cache.hasOwnProperty(cacheKey)) {
// your expensive computation goes here
// to reference the paramaters passed, use arguments[n]
// eg.: result = arguments[0] * arguments[1];
functionMemoized.cache[cacheKey] = result;
}
return functionMemoized.cache[cacheKey];
};
functionMemoized.cache = {};
return functionMemoized;
}());

Why do you need an object with an index. Just store the result/key.
var m = function(func) {
var result=null;
return function() {
if (result===null) {
result = func.apply(this, arguments);
}
return result;
}
};
But I am not sure that is what you want. If the function returns different values based on arguments, than you want to use a key based off the arguments.
var m = function(func) {
var results = {};
return function() {
var key = [].slice.call(arguments).join("-");
if (results[key]===undefined) {
results[key] = func.apply(this, arguments);
}
return results[key];
}
};
var multiply = function (a,b) {
return a * b;
}
var mult = m(multiply);
console.log(mult(2,5)); //runs calculation
console.log(mult(2,5)); //uses cache

If you send the value of the function as a string, you can use that as the index with one minor modification
var m = function(func, scope) {
return function() {
var cached = {};
var index = func; // how do I get or create the index of func.apply(this, func)?
scope = scope || this;
if (!cached[index]) {
func = scope[func]; //Get the reference to the function through the name
cached[index] = func.apply(this, func);
}
return cached[index];
};
};
This does depend on if the index exists in the this object reference. Otherwise you should use a different scope.

jQuery extend multiple objects with the same function

So I'm using $.extend to combine multiple components (objects). Some of these components have a function with the same key. I want the final extended object to have that same key, but have it point to a function which calls all of the merged components' versions of the functions one after the other.
So it'd look something like this:
var a = { foo: function() { console.log("a"); } };
var b = { foo: function() { console.log("b"); } };
var c = {}; // Doesn't have foo
var d = $.extend({}, a, b, c);
var e = $.extend({}, a, c);
var f = $.extend({}, c);
d.foo(); // Should call function() { console.log("a"); console.log("b"); }
e.foo(); // Should call function() { console.log("a"); }
f.foo(); // Should call function() {}
Is there a pragmatic way of doing this? I only want to do this for a specific set of keys, so I would only want to merge those specific keys' functions together and let the ordering in extend overwrite anything else.
Hopefully that makes sense :S

Note
f.foo(); // Should call function() {}
object c does not appear to have property foo . callling f.foo() returns TypeError: undefined is not a function . Not certain if requirement to add foo function to extended f object , or return object c (empty object) from anonymous function ? At piece below , foo function not added to extended f object.
jquery $.Callbacks() utilized to add functions having foo property at $.each()
Try
var a = { foo: function() { console.log("a"); } };
var b = { foo: function() { console.log("b"); } };
var c = {}; // Doesn't have foo
//d.foo();
// Should call function() { console.log("a"); console.log("b"); }
//e.foo();
// Should call function() { console.log("a"); }
//f.foo();
// Should call function() {}
var callbacks = $.Callbacks();
var arr = [], d, e, f;
$.each([a,b,c], function(k, v, j) {
var j = [a,b,c];
// filter objects having `foo` property
if (v.hasOwnProperty("foo")) {
arr.push([v, v.foo]);
if (arr.length > 1) {
callbacks.add(arr[0][1], arr[1][1]);
// `add` `foo` properties to `callbacks`
// `fire` both `callbacks` when `object.foo` called
j[k -1].foo = callbacks.fire;
d = $.extend({}, j[k - 1])
} else {
// `else` extend original data (`fn`, `object`)
// contained within object
e = $.extend({}, j[k + 1]);
f = $.extend({}, j[++k + 1]);
}
}
});
d.foo(); // `a` , `b`
e.foo(); // `b`
console.log(f); // `Object {}`
f.foo() // `TypeError: undefined is not a function`
jsfiddle http://jsfiddle.net/guest271314/3k35buc1/
See jQuery.Callbacks()

Here's what I ended up with based off of guest271314's answer. toMix is the array of components to be mixed into the object. I actually didn't need the dummy functions I thought I might, and I ended up using an array of functions instead of the $.Callbacks() so that I could control the order in which the functions are called. I also needed to use the call() function so that I could call the functions from the correct this object.
this.functionMerge = function(toMix) {
var callbacks = {};
var functions = {};
var obj = {};
var keys = [
'componentWillMount',
'componentDidMount',
'componentWillUpdate',
'componentDidUpdate',
'componentWillUnmount'
]
$.each(keys, function(key, value) {
callbacks[value] = [];
});
for (i = 0; i < toMix.length; ++i) {
$.each(keys, function(key, value) {
if (toMix[i].hasOwnProperty(value) && typeof toMix[i][value] == 'function') {
callbacks[value].push(toMix[i][value]);
}
});
$.extend(true, obj, toMix[i]);
}
$.each(keys, function(key, value) {
functions[value] = function() {
var that = this;
$.each(callbacks[value], function(key, value) {
value.call(that);
});
};
});
return $.extend(true, obj, functions);
}

Currying with functions that take unlimited arguments

Lets say i have the following add function that takes an unlimited number of arguments.
function add () {
var total = 0;
var args = Array.prototype.slice.call(arguments, 0);
for (var i=0; i<args.length; i++) {
total += arguments[i];
}
return total;
}
and the following curry function.
function curryFunction(orig_func) {
var ap = Array.prototype;
var args = arguments;
function fn() {
if (arguments.length != 0) {
ap.push.apply(fn.args, arguments);
return fn;
} else {
return orig_func.apply(this, fn.args);
}
};
return function() {
fn.args = ap.slice.call( args, 1 );
return fn.apply( this, arguments );
};
}
I then want to do something like:
var f = curryFunction(add);
var a = f(3)(4)(3);
var b = f(10)(3);
var result1 = a(); // returns 10
var result2 = b(); // returns 13
However i always get 13 for both a() and b() i assume is because in line
fn.args = ap.slice.call(args, 1);
the existing array [3,4,3] is overwriting with []. Can someone please provide me with a hint on how to make this work? Thanks

The problem is that fn is scoped to curryFunction and so is shared between a and b.
All you have to do is move the definition of fn into the anonymous return function. It's then created when you call f, and the problematic fn.args = line is only called once.
Proof: jsFiddle.

Currying a function which takes indefinitely many arguments can be implemented as follows;
Lets say we have a function called addAll() which returns the sum of all provided arguments.
var addall = (...a) => a.reduce((p,c) => p + c);
And we have a curry function which takes a function and returns curried version ad infinitum up until the returned function is called with no arguments, only when the result of all previously provided arguments will be returned. OK here is the curry function.
var curry = f => (...a) => a.length ? curry(f.bind(f,...a))
: f();
Lets see it in action;
var addAll = (...a) => a.reduce((p,c) => p + c),
curry = f => (...a) => a.length ? curry(f.bind(f,...a)) : f(),
curried = curry(addAll),
result = curried(10,11)(10)(37)(10,17,42)();
console.log(result);
result = curried("a","b")("c")("d")("e","f","g")();
console.log(result);

Set length property of JavaScript object

Let's say I have a JavaScript object:
function a(){
var A = [];
this.length = function(){
return A.length;
};
this.add = function(x){
A.push(x);
};
this.remove = function(){
return A.pop();
};
};
I can use it like so:
var x = new a();
x.add(3);
x.add(4);
alert(x.length()); // 2
alert(x.remove()); // 4
alert(x.length()); // 1
I was trying to make .length not a function, so I could access it like this: x.length, but I've had no luck in getting this to work.
I tried this, but it outputs 0, because that's the length of A at the time:
function a(){
var A = [];
this.length = A.length;
//rest of the function...
};
I also tried this, and it also outputs 0:
function a(){
var A = [];
this.length = function(){
return A.length;
}();
//rest of the function...
};
How do I get x.length to output the correct length of the array inside in the object?

You could use the valueOf hack:
this.length = {
'valueOf': function (){
return A.length;
},
'toString': function (){
return A.length;
}
};
Now you can access the length as x.length. (Although, maybe it's just me, but to me, something about this method feels very roundabout, and it's easy enough to go with a sturdier solution and, for example, update the length property after every modification.)

If you want A to stay 'private', you need to update the public length property on every operation which modifies A's length so that you don't need a method which checks when asked. I would do so via 'private' method.
Code:
var a = function(){
var instance, A, updateLength;
instance = this;
A = [];
this.length = 0;
updateLength = function()
{
instance.length = A.length;
}
this.add = function(x){
A.push(x);
updateLength();
};
this.remove = function(){
var popped = A.pop();
updateLength();
return popped;
};
};
Demo:
http://jsfiddle.net/JAAulde/VT4bb/

Because when you call a.length, you're returning a function. In order to return the output you have to actually invoke the function, i.e.: a.length().
As an aside, if you don't want to have the length property be a function but the actual value, you will need to modify your object to return the property.
function a() {
var A = [];
this.length = 0;
this.add = function(x) {
A.push(x);
this.length = A.length;
};
this.remove = function() {
var removed = A.pop();
this.length = A.length;
return removed;
};
};

While what everyone has said is true about ES3, that length must be a function (otherwise it's value will remain static, unless you hack it to be otherwise), you can have what you want in ES5 (try this in chrome for example):
function a(){
var A = [],
newA = {
get length(){ return A.length;}
};
newA.add = function(x){
A.push(x);
};
newA.remove = function(){
return A.pop();
};
return newA;
}
var x = a();
x.add(3);
x.add(4);
alert(x.length); // 2
alert(x.remove()); // 4
alert(x.length); // 1
You should probably use Object.create instead of the function a, although I've left it as a function to look like your original.

I don't think you can access it as a variable as a variable to my knoledge cannot return the value of a method, unless you will hijack the array object and start hacking in an update of your variable when the push/pop methods are called (ugly!). In order to make your method version work I think you should do the following:
function a(){
this.A = [];
this.length = function(){
return this.A.length;
};
this.add = function(x){
this.A.push(x);
};
this.remove = function(){
return this.A.pop();
};
};

These days you can use defineProperty:
let x = {}
Object.defineProperty(x, 'length', {
get() {
return Object.keys(this).length
},
})
x.length // 0
x.foo = 'bar'
x.length // 1
Or in your specific case:
Object.defineProperty(x, 'length', {
get() {
return A.length
}
})

function a(){
this.A = [];
this.length = function(){
return this.A.length;
};
this.add = function(x){
this.A.push(x);
};
this.remove = function(){
return this.A.pop();
};
};

new Function() with variable parameters

I need to create a function with variable number of parameters using new Function() constructor. Something like this:
args = ['a', 'b'];
body = 'return(a + b);';
myFunc = new Function(args, body);
Is it possible to do it without eval()?
Thank you very much, guys! Actually, a+b was not my primary concern. I'm working on a code which would process and expand templates and I needed to pass unknown (and variable) number of arguments into the function so that they would be introduced as local variables.
For example, if a template contains:
<span> =a </span>
I need to output the value of parameter a. That is, if user declared expanding function as
var expand = tplCompile('template', a, b, c)
and then calls
expand(4, 2, 1)
I need to substitute =a with 4. And yes, I'm well aware than Function is similar to eval() and runs very slow but I don't have any other choice.

You can do this using apply():
args = ['a', 'b', 'return(a + b);'];
myFunc = Function.apply(null, args);
Without the new operator, Function gives exactly the same result. You can use array functions like push(), unshift() or splice() to modify the array before passing it to apply.
You can also just pass a comma-separated string of arguments to Function:
args = 'a, b';
body = 'return(a + b);';
myFunc = new Function(args, body);
On a side note, are you aware of the arguments object? It allows you to get all the arguments passed into a function using array-style bracket notation:
myFunc = function () {
var total = 0;
for (var i=0; i < arguments.length; i++)
total += arguments[i];
return total;
}
myFunc(a, b);
This would be more efficient than using the Function constructor, and is probably a much more appropriate method of achieving what you need.

#AndyE's answer is correct if the constructor doesn't care whether you use the new keyword or not. Some functions are not as forgiving.
If you find yourself in a scenario where you need to use the new keyword and you need to send a variable number of arguments to the function, you can use this
function Foo() {
this.numbers = [].slice.apply(arguments);
};
var args = [1,2,3,4,5]; // however many you want
var f = Object.create(Foo.prototype);
Foo.apply(f, args);
f.numbers; // [1,2,3,4,5]
f instanceof Foo; // true
f.constructor.name; // "Foo"
ES6 and beyond!
// yup, that easy
function Foo (...numbers) {
this.numbers = numbers
}
// use Reflect.construct to call Foo constructor
const f =
Reflect.construct (Foo, [1, 2, 3, 4, 5])
// everything else works
console.log (f.numbers) // [1,2,3,4,5]
console.log (f instanceof Foo) // true
console.log (f.constructor.name) // "Foo"

You can do this:
let args = '...args'
let body = 'let [a, b] = args;return a + b'
myFunc = new Function(args, body);
console.log(myFunc(1, 2)) //3

If you're just wanting a sum(...) function:
function sum(list) {
var total = 0, nums;
if (arguments.length === 1 && list instanceof Array) {
nums = list;
} else {
nums = arguments;
}
for (var i=0; i < nums.length; i++) {
total += nums[i];
}
return total;
}
Then,
sum() === 0;
sum(1) === 1;
sum([1, 2]) === 3;
sum(1, 2, 3) === 6;
sum([-17, 93, 2, -841]) === -763;
If you want more, could you please provide more detail? It's rather difficult to say how you can do something if you don't know what you're trying to do.

A new feature introduced in ES5 is the reduce method of arrays. You can use it to sum numbers, and it is possible to use the feature in older browsers with some compatibility code.

There's a few different ways you could write that.
// assign normally
var ab = ['a','b'].join('');
alert(ab);
// assign with anonymous self-evaluating function
var cd = (function(c) {return c.join("");})(['c','d']);
alert(cd);
// assign with function declaration
function efFunc(c){return c.join("");}
var efArray = ['e','f'];
var ef = efFunc(efArray);
alert(ef);
// assign with function by name
var doFunc = function(a,b) {return window[b](a);}
var ghArray = ['g','h'];
var ghFunc = function(c){return c.join("");}
var gh = doFunc(ghArray,'ghFunc');
alert(gh);
// assign with Class and lookup table
var Function_ = function(a,b) {
this.val = '';
this.body = b.substr(0,b.indexOf('('));
this.args = b.substr(b.indexOf('(')+1,b.lastIndexOf(')')-b.indexOf('(')-1);
switch (this.body) {
case "return":
switch (this.args) {
case "a + b": this.val = a.join(''); break;
}
break;
}
}
var args = ['i', 'j'];
var body = 'return(a + b);';
var ij = new Function_(args, body);
alert(ij.val);

Maybe you want an annoymous function to call an arbitary function.
// user string function
var userFunction = 'function x(...args) { return args.length}';
Wrap it
var annoyFn = Function('return function x(...args) { return args.length}')()
// now call it
annoyFn(args)

new Function(...)
Declaring function in this way causes
the function not to be compiled, and
is potentially slower than the other
ways of declaring functions.
Let is examine it with JSLitmus and run a small test script:
<script src="JSLitmus.js"></script>
<script>
JSLitmus.test("new Function ... ", function() {
return new Function("for(var i=0; i<100; i++) {}");
});
JSLitmus.test("function() ...", function() {
return (function() { for(var i=0; i<100; i++) {} });
});
</script>
What I did above is create a function expression and function constructor performing same operation. The result is as follows:
FireFox Performance Result
IE Performance Result
Based on facts I recommend to use function expression instead of function constructor
var a = function() {
var result = 0;
for(var index=0; index < arguments.length; index++) {
result += arguments[index];
}
return result;
}
alert(a(1,3));

function construct(){
this.subFunction=function(a,b){
...
}
}
var globalVar=new construct();
vs.
var globalVar=new function (){
this.subFunction=function(a,b){
...
}
}
I prefer the second version if there are sub functions.

the b.apply(null, arguments) does not work properly when b inherits a prototype, because 'new' being omitted, the base constructor is not invoked.

In this sample i used lodash:
function _evalExp(exp, scope) {
const k = [null].concat(_.keys(scope));
k.push('return '+exp);
const args = _.map(_.keys(scope), function(a) {return scope[a];});
const func = new (Function.prototype.bind.apply(Function, k));
return func.apply(func, args);
}
_evalExp('a+b+c', {a:10, b:20, c:30});