Classical (non-js) approach to overloading:
function myFunc(){
//code
}
function myFunc(overloaded){
//other code
}
Javascript wont let more than one function be defined with the same name. As such, things like this show up:
function myFunc(options){
if(options["overloaded"]){
//code
}
}
Is there a better workaround for function overloading in javascript other than passing an object with the overloads in it?
Passing in overloads can quickly cause a function to become too verbose because each possible overload would then need a conditional statement. Using functions to accomplish the //code inside of those conditional statements can cause tricky situations with scopes.
There are multiple aspects to argument overloading in Javascript:
Variable arguments - You can pass different sets of arguments (in both type and quantity) and the function will behave in a way that matches the arguments passed to it.
Default arguments - You can define a default value for an argument if it is not passed.
Named arguments - Argument order becomes irrelevant and you just name which arguments you want to pass to the function.
Below is a section on each of these categories of argument handling.
Variable Arguments
Because javascript has no type checking on arguments or required qty of arguments, you can just have one implementation of myFunc() that can adapt to what arguments were passed to it by checking the type, presence or quantity of arguments.
jQuery does this all the time. You can make some of the arguments optional or you can branch in your function depending upon what arguments are passed to it.
In implementing these types of overloads, you have several different techniques you can use:
You can check for the presence of any given argument by checking to see if the declared argument name value is undefined.
You can check the total quantity or arguments with arguments.length.
You can check the type of any given argument.
For variable numbers of arguments, you can use the arguments pseudo-array to access any given argument with arguments[i].
Here are some examples:
Let's look at jQuery's obj.data() method. It supports four different forms of usage:
obj.data("key");
obj.data("key", value);
obj.data();
obj.data(object);
Each one triggers a different behavior and, without using this dynamic form of overloading, would require four separate functions.
Here's how one can discern between all these options in English and then I'll combine them all in code:
// get the data element associated with a particular key value
obj.data("key");
If the first argument passed to .data() is a string and the second argument is undefined, then the caller must be using this form.
// set the value associated with a particular key
obj.data("key", value);
If the second argument is not undefined, then set the value of a particular key.
// get all keys/values
obj.data();
If no arguments are passed, then return all keys/values in a returned object.
// set all keys/values from the passed in object
obj.data(object);
If the type of the first argument is a plain object, then set all keys/values from that object.
Here's how you could combine all of those in one set of javascript logic:
// method declaration for .data()
data: function(key, value) {
if (arguments.length === 0) {
// .data()
// no args passed, return all keys/values in an object
} else if (typeof key === "string") {
// first arg is a string, look at type of second arg
if (typeof value !== "undefined") {
// .data("key", value)
// set the value for a particular key
} else {
// .data("key")
// retrieve a value for a key
}
} else if (typeof key === "object") {
// .data(object)
// set all key/value pairs from this object
} else {
// unsupported arguments passed
}
},
The key to this technique is to make sure that all forms of arguments you want to accept are uniquely identifiable and there is never any confusion about which form the caller is using. This generally requires ordering the arguments appropriately and making sure that there is enough uniqueness in the type and position of the arguments that you can always tell which form is being used.
For example, if you have a function that takes three string arguments:
obj.query("firstArg", "secondArg", "thirdArg");
You can easily make the third argument optional and you can easily detect that condition, but you cannot make only the second argument optional because you can't tell which of these the caller means to be passing because there is no way to identify if the second argument is meant to be the second argument or the second argument was omitted so what's in the second argument's spot is actually the third argument:
obj.query("firstArg", "secondArg");
obj.query("firstArg", "thirdArg");
Since all three arguments are the same type, you can't tell the difference between different arguments so you don't know what the caller intended. With this calling style, only the third argument can be optional. If you wanted to omit the second argument, it would have to be passed as null (or some other detectable value) instead and your code would detect that:
obj.query("firstArg", null, "thirdArg");
Here's a jQuery example of optional arguments. both arguments are optional and take on default values if not passed:
clone: function( dataAndEvents, deepDataAndEvents ) {
dataAndEvents = dataAndEvents == null ? false : dataAndEvents;
deepDataAndEvents = deepDataAndEvents == null ? dataAndEvents : deepDataAndEvents;
return this.map( function () {
return jQuery.clone( this, dataAndEvents, deepDataAndEvents );
});
},
Here's a jQuery example where the argument can be missing or any one of three different types which gives you four different overloads:
html: function( value ) {
if ( value === undefined ) {
return this[0] && this[0].nodeType === 1 ?
this[0].innerHTML.replace(rinlinejQuery, "") :
null;
// See if we can take a shortcut and just use innerHTML
} else if ( typeof value === "string" && !rnoInnerhtml.test( value ) &&
(jQuery.support.leadingWhitespace || !rleadingWhitespace.test( value )) &&
!wrapMap[ (rtagName.exec( value ) || ["", ""])[1].toLowerCase() ] ) {
value = value.replace(rxhtmlTag, "<$1></$2>");
try {
for ( var i = 0, l = this.length; i < l; i++ ) {
// Remove element nodes and prevent memory leaks
if ( this[i].nodeType === 1 ) {
jQuery.cleanData( this[i].getElementsByTagName("*") );
this[i].innerHTML = value;
}
}
// If using innerHTML throws an exception, use the fallback method
} catch(e) {
this.empty().append( value );
}
} else if ( jQuery.isFunction( value ) ) {
this.each(function(i){
var self = jQuery( this );
self.html( value.call(this, i, self.html()) );
});
} else {
this.empty().append( value );
}
return this;
},
Named Arguments
Other languages (like Python) allow one to pass named arguments as a means of passing only some arguments and making the arguments independent of the order they are passed in. Javascript does not directly support the feature of named arguments. A design pattern that is commonly used in its place is to pass a map of properties/values. This can be done by passing an object with properties and values or in ES6 and above, you could actually pass a Map object itself.
Here's a simple ES5 example:
jQuery's $.ajax() accepts a form of usage where you just pass it a single parameter which is a regular Javascript object with properties and values. Which properties you pass it determine which arguments/options are being passed to the ajax call. Some may be required, many are optional. Since they are properties on an object, there is no specific order. In fact, there are more than 30 different properties that can be passed on that object, only one (the url) is required.
Here's an example:
$.ajax({url: "http://www.example.com/somepath", data: myArgs, dataType: "json"}).then(function(result) {
// process result here
});
Inside of the $.ajax() implementation, it can then just interrogate which properties were passed on the incoming object and use those as named arguments. This can be done either with for (prop in obj) or by getting all the properties into an array with Object.keys(obj) and then iterating that array.
This technique is used very commonly in Javascript when there are large numbers of arguments and/or many arguments are optional. Note: this puts an onus on the implementating function to make sure that a minimal valid set of arguments is present and to give the caller some debug feedback what is missing if insufficient arguments are passed (probably by throwing an exception with a helpful error message).
In an ES6 environment, it is possible to use destructuring to create default properties/values for the above passed object. This is discussed in more detail in this reference article.
Here's one example from that article:
function selectEntries({ start=0, end=-1, step=1 } = {}) {
···
};
Then, you can call this like any of these:
selectEntries({start: 5});
selectEntries({start: 5, end: 10});
selectEntries({start: 5, end: 10, step: 2});
selectEntries({step: 3});
selectEntries();
The arguments you do not list in the function call will pick up their default values from the function declaration.
This creates default properties and values for the start, end and step properties on an object passed to the selectEntries() function.
Default values for function arguments
In ES6, Javascript adds built-in language support for default values for arguments.
For example:
function multiply(a, b = 1) {
return a*b;
}
multiply(5); // 5
Further description of the ways this can be used here on MDN.
Overloading a function in JavaScript can be done in many ways. All of them involve a single master function that either performs all the processes, or delegates to sub-functions/processes.
One of the most common simple techniques involves a simple switch:
function foo(a, b) {
switch (arguments.length) {
case 0:
//do basic code
break;
case 1:
//do code with `a`
break;
case 2:
default:
//do code with `a` & `b`
break;
}
}
A more elegant technique would be to use an array (or object if you're not making overloads for every argument count):
fooArr = [
function () {
},
function (a) {
},
function (a,b) {
}
];
function foo(a, b) {
return fooArr[arguments.length](a, b);
}
That previous example isn't very elegant, anyone could modify fooArr, and it would fail if someone passes in more than 2 arguments to foo, so a better form would be to use a module pattern and a few checks:
var foo = (function () {
var fns;
fns = [
function () {
},
function (a) {
},
function (a, b) {
}
];
function foo(a, b) {
var fnIndex;
fnIndex = arguments.length;
if (fnIndex > foo.length) {
fnIndex = foo.length;
}
return fns[fnIndex].call(this, a, b);
}
return foo;
}());
Of course your overloads might want to use a dynamic number of parameters, so you could use an object for the fns collection.
var foo = (function () {
var fns;
fns = {};
fns[0] = function () {
};
fns[1] = function (a) {
};
fns[2] = function (a, b) {
};
fns.params = function (a, b /*, params */) {
};
function foo(a, b) {
var fnIndex;
fnIndex = arguments.length;
if (fnIndex > foo.length) {
fnIndex = 'params';
}
return fns[fnIndex].apply(this, Array.prototype.slice.call(arguments));
}
return foo;
}());
My personal preference tends to be the switch, although it does bulk up the master function. A common example of where I'd use this technique would be a accessor/mutator method:
function Foo() {} //constructor
Foo.prototype = {
bar: function (val) {
switch (arguments.length) {
case 0:
return this._bar;
case 1:
this._bar = val;
return this;
}
}
}
You cannot do method overloading in strict sense. Not like the way it is supported in java or c#.
The issue is that JavaScript does NOT natively support method overloading. So, if it sees/parses two or more functions with a same names it’ll just consider the last defined function and overwrite the previous ones.
One of the way I think is suitable for most of the case is follows -
Lets say you have method
function foo(x)
{
}
Instead of overloading method which is not possible in javascript you can define a new method
fooNew(x,y,z)
{
}
and then modify the 1st function as follows -
function foo(x)
{
if(arguments.length==2)
{
return fooNew(arguments[0], arguments[1]);
}
}
If you have many such overloaded method consider using switch than just if-else statements.
(more details)
PS: Above link goes to my personal blog that has additional details on this.
I am using a bit different overloading approach based on arguments number.
However i believe John Fawcett's approach is also good.
Here the example, code based on John Resig's (jQuery's Author) explanations.
// o = existing object, n = function name, f = function.
function overload(o, n, f){
var old = o[n];
o[n] = function(){
if(f.length == arguments.length){
return f.apply(this, arguments);
}
else if(typeof o == 'function'){
return old.apply(this, arguments);
}
};
}
usability:
var obj = {};
overload(obj, 'function_name', function(){ /* what we will do if no args passed? */});
overload(obj, 'function_name', function(first){ /* what we will do if 1 arg passed? */});
overload(obj, 'function_name', function(first, second){ /* what we will do if 2 args passed? */});
overload(obj, 'function_name', function(first,second,third){ /* what we will do if 3 args passed? */});
//... etc :)
I tried to develop an elegant solution to this problem described here. And you can find the demo here. The usage looks like this:
var out = def({
'int': function(a) {
alert('Here is int '+a);
},
'float': function(a) {
alert('Here is float '+a);
},
'string': function(a) {
alert('Here is string '+a);
},
'int,string': function(a, b) {
alert('Here is an int '+a+' and a string '+b);
},
'default': function(obj) {
alert('Here is some other value '+ obj);
}
});
out('ten');
out(1);
out(2, 'robot');
out(2.5);
out(true);
The methods used to achieve this:
var def = function(functions, parent) {
return function() {
var types = [];
var args = [];
eachArg(arguments, function(i, elem) {
args.push(elem);
types.push(whatis(elem));
});
if(functions.hasOwnProperty(types.join())) {
return functions[types.join()].apply(parent, args);
} else {
if (typeof functions === 'function')
return functions.apply(parent, args);
if (functions.hasOwnProperty('default'))
return functions['default'].apply(parent, args);
}
};
};
var eachArg = function(args, fn) {
var i = 0;
while (args.hasOwnProperty(i)) {
if(fn !== undefined)
fn(i, args[i]);
i++;
}
return i-1;
};
var whatis = function(val) {
if(val === undefined)
return 'undefined';
if(val === null)
return 'null';
var type = typeof val;
if(type === 'object') {
if(val.hasOwnProperty('length') && val.hasOwnProperty('push'))
return 'array';
if(val.hasOwnProperty('getDate') && val.hasOwnProperty('toLocaleTimeString'))
return 'date';
if(val.hasOwnProperty('toExponential'))
type = 'number';
if(val.hasOwnProperty('substring') && val.hasOwnProperty('length'))
return 'string';
}
if(type === 'number') {
if(val.toString().indexOf('.') > 0)
return 'float';
else
return 'int';
}
return type;
};
In javascript you can implement the function just once and invoke the function without the parameters myFunc() You then check to see if options is 'undefined'
function myFunc(options){
if(typeof options != 'undefined'){
//code
}
}
https://github.com/jrf0110/leFunc
var getItems = leFunc({
"string": function(id){
// Do something
},
"string,object": function(id, options){
// Do something else
},
"string,object,function": function(id, options, callback){
// Do something different
callback();
},
"object,string,function": function(options, message, callback){
// Do something ca-raaaaazzzy
callback();
}
});
getItems("123abc"); // Calls the first function - "string"
getItems("123abc", {poop: true}); // Calls the second function - "string,object"
getItems("123abc", {butt: true}, function(){}); // Calls the third function - "string,object,function"
getItems({butt: true}, "What what?" function(){}); // Calls the fourth function - "object,string,function"
No Problem with Overloading in JS , The pb how to maintain a clean code when overloading function ?
You can use a forward to have clean code, based on two things:
Number of arguments (when calling the function).
Type of arguments (when calling the function)
function myFunc(){
return window['myFunc_'+arguments.length+Array.from(arguments).map((arg)=>typeof arg).join('_')](...arguments);
}
/** one argument & this argument is string */
function myFunc_1_string(){
}
//------------
/** one argument & this argument is object */
function myFunc_1_object(){
}
//----------
/** two arguments & those arguments are both string */
function myFunc_2_string_string(){
}
//--------
/** Three arguments & those arguments are : id(number),name(string), callback(function) */
function myFunc_3_number_string_function(){
let args=arguments;
new Person(args[0],args[1]).onReady(args[3]);
}
//--- And so on ....
How about using a proxy (ES6 Feature)?
I didn't find anywhere mentioning this method of doing it. It might be impractical but it's an interesting way nonetheless.
It's similar to Lua's metatables, where you can "overload" the call operator with the __call metamethod in order to achieve overloading.
In JS, it can be done with the apply method in a Proxy handler. You can check the arguments' existence, types, etc. inside the said method, without having to do it in the actual function.
MDN: proxy apply method
function overloads() {}
overloads.overload1 = (a, b) => {
return a + b;
}
overloads.overload2 = (a, b, c) => {
return a + b + c;
}
const overloadedFn = new Proxy(overloads, { // the first arg needs to be an Call-able object
apply(target, thisArg, args) {
if (args[2]) {
return target.overload2(...args);
}
return target.overload1(...args);
}
})
console.log(overloadedFn(1, 2, 3)); // 6
console.log(overloadedFn(1, 2)); // 3
Check this out:
http://www.codeproject.com/Articles/688869/Overloading-JavaScript-Functions
Basically in your class, you number your functions that you want to be overloaded and then with one function call you add function overloading, fast and easy.
Since JavaScript doesn't have function overload options object can be used instead. If there are one or two required arguments, it's better to keep them separate from the options object. Here is an example on how to use options object and populated values to default value in case if value was not passed in options object.
function optionsObjectTest(x, y, opts) {
opts = opts || {}; // default to an empty options object
var stringValue = opts.stringValue || "string default value";
var boolValue = !!opts.boolValue; // coerces value to boolean with a double negation pattern
var numericValue = opts.numericValue === undefined ? 123 : opts.numericValue;
return "{x:" + x + ", y:" + y + ", stringValue:'" + stringValue + "', boolValue:" + boolValue + ", numericValue:" + numericValue + "}";
}
here is an example on how to use options object
For this you need to create a function that adds the function to an object, then it will execute depending on the amount of arguments you send to the function:
<script >
//Main function to add the methods
function addMethod(object, name, fn) {
var old = object[name];
object[name] = function(){
if (fn.length == arguments.length)
return fn.apply(this, arguments)
else if (typeof old == 'function')
return old.apply(this, arguments);
};
}
 var ninjas = {
values: ["Dean Edwards", "Sam Stephenson", "Alex Russell"]
};
//Here we declare the first function with no arguments passed
addMethod(ninjas, "find", function(){
return this.values;
});
//Second function with one argument
addMethod(ninjas, "find", function(name){
var ret = [];
for (var i = 0; i < this.values.length; i++)
if (this.values[i].indexOf(name) == 0)
ret.push(this.values[i]);
return ret;
});
//Third function with two arguments
addMethod(ninjas, "find", function(first, last){
var ret = [];
for (var i = 0; i < this.values.length; i++)
if (this.values[i] == (first + " " + last))
ret.push(this.values[i]);
return ret;
});
//Now you can do:
ninjas.find();
ninjas.find("Sam");
ninjas.find("Dean", "Edwards")
</script>
How about using spread operator as a parameter? The same block can be called with Multiple parameters. All the parameters are added into an array and inside the method you can loop in based on the length.
function mName(...opt){
console.log(opt);
}
mName(1,2,3,4); //[1,2,3,4]
mName(1,2,3); //[1,2,3]
I like to add sub functions within a parent function to achieve the ability to differentiate between argument groups for the same functionality.
var doSomething = function() {
var foo;
var bar;
};
doSomething.withArgSet1 = function(arg0, arg1) {
var obj = new doSomething();
// do something the first way
return obj;
};
doSomething.withArgSet2 = function(arg2, arg3) {
var obj = new doSomething();
// do something the second way
return obj;
};
What you are trying to achieve is best done using the function's local arguments variable.
function foo() {
if (arguments.length === 0) {
//do something
}
if (arguments.length === 1) {
//do something else
}
}
foo(); //do something
foo('one'); //do something else
You can find a better explanation of how this works here.
(() => {
//array that store functions
var Funcs = []
/**
* #param {function} f overload function
* #param {string} fname overload function name
* #param {parameters} vtypes function parameters type descriptor (number,string,object....etc
*/
overloadFunction = function(f, fname, ...vtypes) {
var k,l, n = false;
if (!Funcs.hasOwnProperty(fname)) Funcs[fname] = [];
Funcs[fname].push([f, vtypes?vtypes: 0 ]);
window[fname] = function() {
for (k = 0; k < Funcs[fname].length; k++)
if (arguments.length == Funcs[fname][k][0].length) {
n=true;
if (Funcs[fname][k][1]!=0)
for(i=0;i<arguments.length;i++)
{
if(typeof arguments[i]!=Funcs[fname][k][1][i])
{
n=false;
}
}
if(n) return Funcs[fname][k][0].apply(this, arguments);
}
}
}
})();
//First sum function definition with parameter type descriptors
overloadFunction(function(a,b){return a+b},"sum","number","number")
//Second sum function definition with parameter with parameter type descriptors
overloadFunction(function(a,b){return a+" "+b},"sum","string","string")
//Third sum function definition (not need parameter type descriptors,because no other functions with the same number of parameters
overloadFunction(function(a,b,c){return a+b+c},"sum")
//call first function
console.log(sum(4,2));//return 6
//call second function
console.log(sum("4","2"));//return "4 2"
//call third function
console.log(sum(3,2,5));//return 10
//ETC...
always in the process of learning Javascript and modifying a cool autocomplete library, i am now in front of this :
i need to check if something passed in an object literal is a variable/field (that is to be considered as a simple value) or is something that can be called.
(as MY autocomplete depend on many input fields, i need to "value" the right things, just before the Ajax.Request) so that this declaration (see the 'extra' parts...)
myAutoComplete = new Autocomplete('query', {
serviceUrl:'autoComplete.rails',
minChars:3,
maxHeight:400,
width:300,
deferRequestBy:100,
// callback function:
onSelect: function(value, data){
alert('You selected: ' + value + ', ' + data);
}
// the lines below are the extra part that i add to the library
// an optional parameter, that will handle others arguments to pass
// if needed, these must be value-ed just before the Ajax Request...
, extraParametersForAjaxRequest : {
myExtraID : function() { return document.getElementById('myExtraID').value; }
}
see the "1 // here i'm lost..." below, and instead of 1 => i would like to check, if extraParametersForAjaxRequest[x] is callable or not, and call it if so, keeping only its value if not. So that, i get the right value of my other inputs... while keeping a really generic approach and clean modification of this library...
{
var ajaxOptions = {
parameters: { query: this.currentValue , },
onComplete: this.processResponse.bind(this),
method: 'get'
};
if (this.options.hasOwnProperty('extraParametersForAjaxRequest'))
{
for (var x in this.options.extraParametersForAjaxRequest)
{
ajaxOptions.parameters[x] = 1 // here i'm lost...
}
}
new Ajax.Request(this.serviceUrl, ajaxOptions );
You can do a typeof to see if the parameter is a function, and call it if it is.
var value;
for (var x in this.options.extraParametersForAjaxRequest)
{
value = this.options.extraParametersForAjaxRequest[x];
if (typeof(value) == 'function') {
ajaxOptions.parameters[x] = value();
}
else {
ajaxOptions.parameters[x] = value;
}
}
if (typeof this.options.extraParametersForAjaxRequest[x]==='function') {
}
You should also do this:
if (this.options.extraParametersForAjaxRequest.hasOwnProperty(x) {
if (typeof this.options.extraParametersForAjaxRequest[x]==='function') {
}
}
when iterating through properties of objects, otherwise you can end up looking at prototype members too.
Another suggestion is to make this more readable with an alias for the thing you're working with. So the ultimate would be:
var opts = this.options.extraParametersForAjaxRequest;
// don't need to check for existence of property explicitly with hasOwnProperty
// just try to access it, and check to see if the result is
// truthy. if extraParametersForAjaxRequest isn't there, no error will
// result and "opts" will just be undefined
if (opts)
{
for (var x in opts) {
if (opts.hasOwnProperty(x) && typeof opts[x]==='function') {
}
}
}
Take this object:
x = {
"key1": "xxx",
"key2": function(){return this.key1}
}
If I do this:
y = JSON.parse( JSON.stringify(x) );
Then y will return { "key1": "xxx" }. Is there anything one could do to transfer functions via stringify? Creating an object with attached functions is possible with the "ye goode olde eval()", but whats with packing it?
json-stringify-function is a similar post to this one.
A snippet discovered via that post may be useful to anyone stumbling across this answer. It works by making use of the replacer parameter in JSON.stringify and the reviver parameter in JSON.parse.
More specifically, when a value happens to be of type function, .toString() is called on it via the replacer. When it comes time to parse, eval() is performed via the reviver when a function is present in string form.
var JSONfn;
if (!JSONfn) {
JSONfn = {};
}
(function () {
JSONfn.stringify = function(obj) {
return JSON.stringify(obj,function(key, value){
return (typeof value === 'function' ) ? value.toString() : value;
});
}
JSONfn.parse = function(str) {
return JSON.parse(str,function(key, value){
if(typeof value != 'string') return value;
return ( value.substring(0,8) == 'function') ? eval('('+value+')') : value;
});
}
}());
Code Snippet taken from Vadim Kiryukhin's JSONfn.js or see documentation at Home Page
I've had a similar requirement lately. To be clear, the output looks like JSON but in fact is just javascript.
JSON.stringify works well in most cases, but "fails" with functions.
I got it working with a few tricks:
make use of replacer (2nd parameter of JSON.stringify())
use func.toString() to get the JS code for a function
remember which functions have been stringified and replace them directly in the result
And here's how it looks like:
// our source data
const source = {
"aaa": 123,
"bbb": function (c) {
// do something
return c + 1;
}
};
// keep a list of serialized functions
const functions = [];
// json replacer - returns a placeholder for functions
const jsonReplacer = function (key, val) {
if (typeof val === 'function') {
functions.push(val.toString());
return "{func_" + (functions.length - 1) + "}";
}
return val;
};
// regex replacer - replaces placeholders with functions
const funcReplacer = function (match, id) {
return functions[id];
};
const result = JSON
.stringify(source, jsonReplacer) // generate json with placeholders
.replace(/"\{func_(\d+)\}"/g, funcReplacer); // replace placeholders with functions
// show the result
document.body.innerText = result;
body { white-space: pre-wrap; font-family: monospace; }
Important: Be careful about the placeholder format - make sure it's not too generic. If you change it, also change the regex as applicable.
Technically this is not JSON, I can also hardly imagine why would you want to do this, but try the following hack:
x.key2 = x.key2.toString();
JSON.stringify(x) //"{"key1":"xxx","key2":"function (){return this.key1}"}"
Of course the first line can be automated by iterating recursively over the object. Reverse operation is harder - function is only a string, eval will work, but you have to guess whether a given key contains a stringified function code or not.
You can't pack functions since the data they close over is not visible to any serializer.
Even Mozilla's uneval cannot pack closures properly.
Your best bet, is to use a reviver and a replacer.
https://yuilibrary.com/yui/docs/json/json-freeze-thaw.html
The reviver function passed to JSON.parse is applied to all key:value pairs in the raw parsed object from the deepest keys to the highest level. In our case, this means that the name and discovered properties will be passed through the reviver, and then the object containing those keys will be passed through.
This is what I did https://gist.github.com/Lepozepo/3275d686bc56e4fb5d11d27ef330a8ed
function stringifyWithFunctions(object) {
return JSON.stringify(object, (key, val) => {
if (typeof val === 'function') {
return `(${val})`; // make it a string, surround it by parenthesis to ensure we can revive it as an anonymous function
}
return val;
});
};
function parseWithFunctions(obj) {
return JSON.parse(obj, (k, v) => {
if (typeof v === 'string' && v.indexOf('function') >= 0) {
return eval(v);
}
return v;
});
};
The naughty but effective way would be to simply:
Function.prototype.toJSON = function() { return this.toString(); }
Though your real problem (aside from modifying the prototype of Function) would be deserialization without the use of eval.
I have come up with this solution which will take care of conversion of functions (no eval). All you have to do is put this code before you use JSON methods. Usage is exactly the same but right now it takes only one param value to convert to a JSON string, so if you pass remaning replacer and space params, they will be ignored.
void function () {
window.JSON = Object.create(JSON)
JSON.stringify = function (obj) {
return JSON.__proto__.stringify(obj, function (key, value) {
if (typeof value === 'function') {
return value.toString()
}
return value
})
}
JSON.parse = function (obj) {
return JSON.__proto__.parse(obj, function (key, value) {
if (typeof value === 'string' && value.slice(0, 8) == 'function') {
return Function('return ' + value)()
}
return value
})
}
}()
// YOUR CODE GOES BELOW HERE
x = {
"key1": "xxx",
"key2": function(){return this.key1}
}
const y = JSON.parse(JSON.stringify(x))
console.log(y.key2())
It is entirely possible to create functions from string without eval()
var obj = {a:function(a,b){
return a+b;
}};
var serialized = JSON.stringify(obj, function(k,v){
//special treatment for function types
if(typeof v === "function")
return v.toString();//we save the function as string
return v;
});
/*output:
"{"a":"function (a,b){\n return a+b;\n }"}"
*/
now some magic to turn string into function with this function
var compileFunction = function(str){
//find parameters
var pstart = str.indexOf('('), pend = str.indexOf(')');
var params = str.substring(pstart+1, pend);
params = params.trim();
//find function body
var bstart = str.indexOf('{'), bend = str.lastIndexOf('}');
var str = str.substring(bstart+1, bend);
return Function(params, str);
}
now use JSON.parse with reviver
var revivedObj = JSON.parse(serialized, function(k,v){
// there is probably a better way to determ if a value is a function string
if(typeof v === "string" && v.indexOf("function") !== -1)
return compileFunction(v);
return v;
});
//output:
revivedObj.a
function anonymous(a,b
/**/) {
return a+b;
}
revivedObj.a(1,2)
3
To my knowledge, there are no serialization libraries that persist functions - in any language. Serialization is what one does to preserve data. Compilation is what one does to preserve functions.
It seems that people landing here are dealing with structures that would be valid JSON if not for the fact that they contain functions. So how do we handle stringifying these structures?
I ran into the problem while writing a script to modify RequireJS configurations. This is how I did it. First, there's a bit of code earlier that makes sure that the placeholder used internally (">>>F<<<") does not show up as a value in the RequireJS configuration. Very unlikely to happen but better safe than sorry. The input configuration is read as a JavaScript Object, which may contain arrays, atomic values, other Objects and functions. It would be straightforwardly stringifiable as JSON if functions were not present. This configuration is the config object in the code that follows:
// Holds functions we encounter.
var functions = [];
var placeholder = ">>>F<<<";
// This handler just records a function object in `functions` and returns the
// placeholder as the value to insert into the JSON structure.
function handler(key, value) {
if (value instanceof Function) {
functions.push(value);
return placeholder;
}
return value;
}
// We stringify, using our custom handler.
var pre = JSON.stringify(config, handler, 4);
// Then we replace the placeholders in order they were encountered, with
// the functions we've recorded.
var post = pre.replace(new RegExp('"' + placeholder + '"', 'g'),
functions.shift.bind(functions));
The post variable contains the final value. This code relies on the fact that the order in which handler is called is the same as the order of the various pieces of data in the final JSON. I've checked the ECMAScript 5th edition, which defines the stringification algorithm and cannot find a case where there would be an ordering problem. If this algorithm were to change in a future edition the fix would be to use unique placholders for function and use these to refer back to the functions which would be stored in an associative array mapping unique placeholders to functions.