I find the named parameters feature in C# quite useful in some cases.
calculateBMI(70, height: 175);
What can I use if I want this in JavaScript?
What I don’t want is this:
myFunction({ param1: 70, param2: 175 });
function myFunction(params){
// Check if params is an object
// Check if the parameters I need are non-null
// Blah blah
}
That approach I’ve already used. Is there another way?
I’m okay using any library to do this.
ES2015 and later
In ES2015, parameter destructuring can be used to simulate named parameters. It would require the caller to pass an object, but you can avoid all of the checks inside the function if you also use default parameters:
myFunction({ param1 : 70, param2 : 175});
function myFunction({param1, param2}={}){
// ...function body...
}
// Or with defaults,
function myFunc({
name = 'Default user',
age = 'N/A'
}={}) {
// ...function body...
}
ES5
There is a way to come close to what you want, but it is based on the output of Function.prototype.toString [ES5], which is implementation dependent to some degree, so it might not be cross-browser compatible.
The idea is to parse the parameter names from the string representation of the function so that you can associate the properties of an object with the corresponding parameter.
A function call could then look like
func(a, b, {someArg: ..., someOtherArg: ...});
where a and b are positional arguments and the last argument is an object with named arguments.
For example:
var parameterfy = (function() {
var pattern = /function[^(]*\(([^)]*)\)/;
return function(func) {
// fails horribly for parameterless functions ;)
var args = func.toString().match(pattern)[1].split(/,\s*/);
return function() {
var named_params = arguments[arguments.length - 1];
if (typeof named_params === 'object') {
var params = [].slice.call(arguments, 0, -1);
if (params.length < args.length) {
for (var i = params.length, l = args.length; i < l; i++) {
params.push(named_params[args[i]]);
}
return func.apply(this, params);
}
}
return func.apply(null, arguments);
};
};
}());
Which you would use as:
var foo = parameterfy(function(a, b, c) {
console.log('a is ' + a, ' | b is ' + b, ' | c is ' + c);
});
foo(1, 2, 3); // a is 1 | b is 2 | c is 3
foo(1, {b:2, c:3}); // a is 1 | b is 2 | c is 3
foo(1, {c:3}); // a is 1 | b is undefined | c is 3
foo({a: 1, c:3}); // a is 1 | b is undefined | c is 3
DEMO
There are some drawbacks to this approach (you have been warned!):
If the last argument is an object, it is treated as a "named argument objects"
You will always get as many arguments as you defined in the function, but some of them might have the value undefined (that's different from having no value at all). That means you cannot use arguments.length to test how many arguments have been passed.
Instead of having a function creating the wrapper, you could also have a function which accepts a function and various values as arguments, such as
call(func, a, b, {posArg: ... });
or even extend Function.prototype so that you could do:
foo.execute(a, b, {posArg: ...});
No - the object approach is JavaScript's answer to this. There is no problem with this provided your function expects an object rather than separate params.
Lots of people say to just use the "Pass an object" trick so that you have named parameters.
/**
* My Function
*
* #param {Object} arg1 Named arguments
*/
function myFunc(arg1) { }
myFunc({ param1 : 70, param2 : 175});
And that works great, except... when it comes to most IDEs out there, a lot of us developers rely on type / argument hints within our IDE. I personally use PhpStorm (along with other JetBrains IDEs, like PyCharm for Python and AppCode for Objective-C).
And the biggest problem with using the "Pass an object" trick is that when you are calling the function, the IDE gives you a single type hint and that's it... How are we supposed to know what parameters and types should go into the arg1 object?
So... the "Pass an object" trick doesn't work for me... It actually causes more headaches with having to look at each function's docblock before I know what parameters the function expects.... Sure, it's great for when you are maintaining existing code, but it's horrible for writing new code.
Well, this is the technique I use... Now, there may be some issues with it, and some developers may tell me I'm doing it wrong, and I have an open mind when it comes to these things... I am always willing to look at better ways of accomplishing a task... So, if there is an issue with this technique, then comments are welcome.
/**
* My Function
*
* #param {string} arg1 Argument 1
* #param {string} arg2 Argument 2
*/
function myFunc(arg1, arg2) { }
var arg1, arg2;
myFunc(arg1='Param1', arg2='Param2');
This way, I have the best of both worlds. New code is easy to write as my IDE gives me all the proper argument hints. And, while maintaining code later on, I can see at a glance, not only the value passed to the function, but also the name of the argument. The only overhead I see is declaring your argument names as local variables to keep from polluting the global namespace. Sure, it's a bit of extra typing, but it's trivial compared to the time it takes to look up docblocks while writing new code or maintaining existing code.
Update - 2022
JavaScript now has the ability to have something close to named parameters using object destructuring available in ES6. Most newer browsers can use this feature See browser support
This is how it works:
// Define your function like this
function myFunc({arg1, arg2, arg3}) {
// Function body
}
// Call your function like this
myFunc({arg1: "value1", arg2: "value2", arg3: "value3"})
// You can also have default values for arguments
function myFunc2({firstName, lastName, age = 21}) {
// Function body
}
// And you can call it with or without an "age" argument
myFunc({firstName: "John", lastName: "Doe"}) // Age will be 21
myFunc({firstName: "Jane", lastName: "Doe", age: 22})
The best part is that most IDE's now support this syntax and you get good argument hint support
TypeScript
For those of you using TypeScript, you can do the same thing using this syntax
function myFunc(
{firstName, lastName, age = 21}:
{firstName: string, lastName: string, age?: number}
) {
// Function body
}
OR, using an interface
interface Params {
firstName: string
lastName: string
age?: number
}
function myFunc({firstName, lastName, age = 21}: Params) {
// Function body
}
If you want to make it clear what each of the parameters are, rather than just calling
someFunction(70, 115);
do the following:
var width = 70, height = 115;
someFunction(width, height);
Sure, it's an extra line of code, but it wins on readability.
Another way would be to use attributes of a suitable object, e.g. like so:
function plus(a,b) { return a+b; };
Plus = { a: function(x) { return { b: function(y) { return plus(x,y) }}},
b: function(y) { return { a: function(x) { return plus(x,y) }}}};
sum = Plus.a(3).b(5);
Of course for this made up example it is somewhat meaningless. But in cases where the function looks like
do_something(some_connection_handle, some_context_parameter, some_value)
it might be more useful. It also could be combined with "parameterfy" idea to create such an object out of an existing function in a generic way. That is for each parameter it would create a member that can evaluate to a partial evaluated version of the function.
This idea is of course related to Schönfinkeling aka Currying.
Calling function f with named parameters passed as the object
o = {height: 1, width: 5, ...}
is basically calling its composition f(...g(o)) where I am using the spread syntax and g is a "binding" map connecting the object values with their parameter positions.
The binding map is precisely the missing ingredient, that can be represented by the array of its keys:
// map 'height' to the first and 'width' to the second param
binding = ['height', 'width']
// take binding and arg object and return aray of args
withNamed = (bnd, o) => bnd.map(param => o[param])
// call f with named args via binding
f(...withNamed(binding, {hight: 1, width: 5}))
Note the three decoupled ingredients: the function, the object with named arguments and the binding. This decoupling allows for a lot of flexibility to use this construct, where the binding can be arbitrarily customized in function's definition and arbitrarily extended at the function call time.
For instance, you may want to abbreviate height and width as h and w inside your function's definition, to make it shorter and cleaner, while you still want to call it with full names for clarity:
// use short params
f = (h, w) => ...
// modify f to be called with named args
ff = o => f(...withNamed(['height', 'width'], o))
// now call with real more descriptive names
ff({height: 1, width: 5})
This flexibility is also more useful for functional programming, where functions can be arbitrarily transformed with their original param names getting lost.
There is another way. If you're passing an object by reference, that object's properties will appear in the function's local scope. I know this works for Safari (haven't checked other browsers) and I don't know if this feature has a name, but the below example illustrates its use.
Although in practice I don't think that this offers any functional value beyond the technique you're already using, it's a little cleaner semantically. And it still requires passing a object reference or an object literal.
function sum({ a:a, b:b}) {
console.log(a+'+'+b);
if(a==undefined) a=0;
if(b==undefined) b=0;
return (a+b);
}
// will work (returns 9 and 3 respectively)
console.log(sum({a:4,b:5}));
console.log(sum({a:3}));
// will not work (returns 0)
console.log(sum(4,5));
console.log(sum(4));
Coming from Python this bugged me. I wrote a simple wrapper/Proxy for node that will accept both positional and keyword objects.
https://github.com/vinces1979/node-def/blob/master/README.md
NB. My answer of 2016 is not correct and misleading as mentioned in comments.
Trying Node-6.4.0 ( process.versions.v8 = '5.0.71.60') and Node Chakracore-v7.0.0-pre8 and then Chrome-52 (V8=5.2.361.49), I've noticed that named parameters are almost implemented, but that order has still precedence. I can't find what the ECMA standard says.
>function f(a=1, b=2){ console.log(`a=${a} + b=${b} = ${a+b}`) }
> f()
a=1 + b=2 = 3
> f(a=5)
a=5 + b=2 = 7
> f(a=7, b=10)
a=7 + b=10 = 17
But order is required!! Is it the standard behaviour?
> f(b=10)
a=10 + b=2 = 12
This is admittedly pseudocode, but I believe it'll work (I know it works in TypeScript; I'm adopting it for JavaScript).
// Target Function
const myFunc = (a=1,b=2,c=3) => {a+b+c}
// Goal usage:
myFunc(a=5, b=6) // 14
myFunc(c=0) // 3
// Set your defaults
const myFuncDefaults = {a:1, b:2, c:3};
// Override them with passed parameters
const myFuncParams = (params) => { return Object.assign(myFuncDefaults, params)}
// Use the overloaded dict as the input
const myFunc2 = (params) => {
let {a, b, c} = myFuncParams(params);
return myFunc(a, b, c)
}
// Usage:
myFunc({a:5, b:6}) // 14
myFunc({c:0}) // 3
// Written more succinctly:
const myFunc = (params) => {
let {a,b,c} = Object.assign({a:1, b:2, c:3}, params)
return a + b + c
}
For what it's worth, TypeScript makes this kind of nice with hinting:
interface IParams {
a: number;
b: number;
c: number;
}
const myFunc = (params: Partial<IParams>): number => {
const default: IParams = {a:1, b:2, c:3};
let {a, b, c} = Object.assign(default, params)
return a + b + c
}
Yes, well, kind of. I've found two solutions. I'll explain just one.
In this solution, we give up positional arguments, though.
We can use an object (almost identical to a dict in Python) to pass the arguments.
In this example, I'm using the function to generate the name of a image file:
// First we define our function with just ONE argument
function name_of_img(img_desc){
// With this step, any undefined value will be assigned a value
if(img_desc.size == undefined) {img_desc.size = "400x500"}
if(img_desc.format == undefined) {img_desc.format = ".png"}
console.log(img_desc.size + img_desc.format)
}
// Notice inside our function we're passing a dict/object
name_of_img({size: "200x250", format : ".jpg"})
// In Python name_of_img(size="200x250" , format="jpg")
// returns "200x250.jpg"
name_of_img({size: "1200x950"})
// In Python name_of_img(size="1200x950")
// returns "1200x950.png"
We can modify this example, so we can use positional arguments too, we can also modify it so non valid arguments can be passed, I think I will make a GitHub repository about this.
Contrary to what is commonly believed, named parameters can be implemented in standard, old-school JavaScript (for boolean parameters only) by means of a simple, neat coding convention, as shown below.
function f(p1=true, p2=false) {
...
}
f(!!"p1"==false, !!"p2"==true); // call f(p1=false, p2=true)
Caveats:
Ordering of arguments must be preserved - but the pattern is still useful, since it makes it obvious which actual argument is meant for which formal parameter without having to grep for the function signature or use an IDE.
This only works for booleans. However, I'm sure a similar pattern could be developed for other types using JavaScript's unique type coercion semantics.
In a program i am writing i need an object variable that looks like this:
var map {
cube: {__pt3arraylocation__:[0,0,0], poly: new Object()},
other: {__pt3arraylocation__:[1,0,0], poly: new Object()}
};
However, i want to be able to type map.cube and have it return the pt3arraylocation as a default unless i specify what i want by typing map.cube.poly
for example: map.cube would return [0,0,0] and map.cube.poly would return the object poly in the object cube
thanks in advance
i want to be able to type map.cube and have it return the pt3arraylocation as a default unless i specify what i want by typing map.cube.poly
for example: map.cube would return [0,0,0] and map.cube.poly would return the object poly in the object cube
You can't do that in JavaScript.
However, as an alternative, it's worth noting that you can add arbitrary properties to arrays if you want to. So for instance:
var map {
cube: [0,0,0],
other: [1,0,0]
};
map.cube.poly = {}; // {} is the same as `new Object()` but not subject to people...
map.other.poly = {}; // ...overriding the `Object` symbol
Then map.cube gives you the array, and map.cube.poly gives you the object you've stored on that array.
How is this possible? Because in JavaScript, arrays aren't really arrays. They're just objects that have an automatic length property, treat a class of property names (all numeric ones) in a special way, and have Array.prototype backing them up. And of course, you can add properties to any object.
There's no literal syntax for doing this, which is why I had to do it with assignments after the object initializer above. But it's perfectly valid. I use it for cross-indexing arrays all the time.
Do be sure, if you do this, that you're not using for..in incorrectly; more.
The best way to do this I would say is like this:
var map {
cube: [0,0,0],
other: [1,0,0]
};
map.cube.poly = new Object();
map.other.poly = new Object();
That is not possible to achive. Maybe you can play around with toString()
var map = {
cube: {
__pt3arraylocation__:[0,0,0],
poly: new Object(),
toString : function() {
return this.__pt3arraylocation__.toString()
}
},
other: {__pt3arraylocation__:[1,0,0], poly: new Object()}
};
map.cube == '0,0,0'
map.cube.split(',') == map.cube.__pt3arraylocation__
There is no way to do that exactly as you want - if you request an object (which map.cube is), you get an object. However, there are a few ways to do something similar.
when used as a parameter to functions or operations that require string, like alert(map.cube) or "sometext" + map.cube, the object is converted to string by calling the toString function. You can therefore define, for example:
map.cube.toString = function() { return this.__pt3arraylocation__.toString(); };
a similar thing happens when there if the object is used in context when a number is needed. In this case, it is converted with valueOf(). So you can use, for example
map.cube.valueOf = function() { return this.__pt3arraylocation__.length; };
or you can obtain the default value via a function call, if you define your object as a function instead
var map = {
cube: function() {
return map.cube.__pt3arraylocation__;
}
}
map.cube.__pt3arraylocation__ = [0,0,0];
map.cube.poly = new Object();
alert(map.cube.__pt3arraylocation__); // 0,0,0
alert(map.cube.poly); // [object Object]
alert(map.cube()); // same as map.cube.__pt3arraylocation__
As you can see, in JavaScript, functions are objects like any other, so you can not only call them, but also set fields and methods to them.
T.J. Crowder is right. Based on his answer I'd like to remark that you can also assign map.cube like this
var map = {
cube: function(){
var val = [0,0,0];
val.poly = {};
return val; }()
};
Or for more values:
function polyval(val){
val.poly = {};
return val;
}
var map = {
cube: polyval([0,0,0]),
other: polyval([1,0,0]),
more: polyval([1,1,0])
/* ... etc */
};
Could anyone explain to me why the code sample below reports true? I would have assumed that like in C# the instance of Test1 != instance of Test2.
Update: So I think I will go with some unique identifier stored in the base of both Test1 and Test2.
function Test1() { };
function Test2() { };
var test1 = new Test1();
var test2 = new Test2();
var dict = new Array();
dict[test1] = true;
alert(dict[test2]);
Your object (JavaScript's hashtable) is not using the instance of test1 or test2, but the string representation, as a key. Since both test1 and test2 have the same string representation: "[object Object]", the true value is associated with that key.
Try doing something like below instead:
function Test1(id) { this.id=id };
function Test2(id) { this.id=id };
var test1 = new Test1('1');
var test2 = new Test2('2');
var dict = {};
dict[test1.id] = true;
console.log(dict[test1.id]);
Keys in 'hashtables' (objects, basically) are always strings. So anything you add will be converted to a string.
new Test1();
returns a instance of Test1. Converted as a string, this is:
"[object Object]"
The same goes for Test2. So in fact, when storing true under the key of new Test1() as a string, you are working with the exact same record as the one by obtaining with the key new Test2() as a string. In other words,
(new Test1()).toString() == (new Test2()).toString();
The actual object is therefore simply:
{
"[object Object]": true
}
A solution is overwriting .toString() like this:
Test1.prototype.toString = function() { return "Test1" };
Test2.prototype.toString = function() { return "Test2" };
Then dict[test1] will be stored as dict['Test1'] and dict[test2] as dict['Test2'], which allows you to differ between them. Still, manually setting dict['Test1'] would overwrite things. As far as I know, there is no way to assign an object as a key.
Javascript objects aren't exactly hashtables; they're actually objects with string keys.
When you use an object as a key, the object is converted to a string by calling toString().
toString() will return the same string for all custom classes (unless you create your own toString), so they end up using the same key.
First: Use arrays only for numerical keys. For anything else use objects.
Property names can only be strings. Anything else is converted to its string representation. In case of objects, this is [object Object] or whatever toString() returns.
Which means, that if you want to make both objects distinguishable, you have to override this method and let it return something which is unique to each instance.
This question might help you: Hash/associative array using several objects as key
while review a javascript coding, i saw that
var detailInf = {
"hTitle":"Results",
"hMark":"98"
};
What's the concept behind this js coding. While give alert for the variable its shows as "[object Object]". So this is an object, then how can we access the variable and reveal the data from this object.
Try doing this:
alert(detailInf['hTitle']);
alert(detailInf.hTitle);
Both will alert "Results" - this is a Javascript object that can be used as a dictionary of sorts.
Required reading: Objects as associative arrays
As a footnote, you should really get Firebug when messing around with Javascript. You could then just console.log(detailInf); and you would get a nicely mapped out display of the object in the console.
That form of a JavaScript object is called an object literal, just like there are array literals. For example, the following two array declarations are identical:
var a = [1, 2, 3]; // array literal
var b = new Array(1, 2, 3); // using the Array constructor
Just as above, an object may be declared in multiple ways. One of them is object literal in which you declare the properties along with the object:
var o = {property: "value"}; // object literal
Is equivalent to:
var o = new Object; // using the Object constructor
o.property = "value";
Objects may also be created from constructor functions. Like so:
var Foo = function() {
this.property = "value";
};
var o = new Foo;
Adding methods
As I said in a comment a few moments ago, this form of declaring a JavaScript object is not a JSON format. JSON is a data format and does not allow functions as values. That means the following is a valid JavaScript object literal, but not a valid JSON format:
var user = {
age : 16,
// this is a method
isAdult : function() {
// the object is referenced by the special variable: this
return this.age >= 18;
}
};
Also, the name of the properties need not be enclosed inside quotes. This is however required in JSON. In JavaScript we enclose them in brackets where the property name is a reserved word, like class, while and others. So the following are also equivalent:
var o = {
property : "value",
};
var o = {
"property" : "value",
};
Further more, the keys may also be numbers:
var a = {
0 : "foo",
1 : "bar",
2 : "abz"
};
alert(a[1]); // bar
Array-like objects
Now, if the above object would have also a length property, it will be an array like object:
var arrayLike = {
0 : "foo",
1 : "bar",
2 : "baz",
length : 3
};
Array-like means it can be easily iterated with normal iteration constructs (for, while). However, you cannot apply array methods on it. Like array.slice(). But this is another topic.
Square Bracket Notation
As Paolo Bergantino already said, you may access an object's properties using both the dot notation, as well as the square bracket notation. For example:
var o = {
property : "value"
};
o.property;
o["property"];
When would you want to use one over the other? People use square bracket notation when the property names is dynamically determined, like so:
var getProperty = function(object, property) {
return object[property];
};
Or when the property name is a JavaScript reserved word, for example while.
object["while"];
object.while; // error
That's an object in JSON format. That's a javascript object literal. Basically, the bits to the left of the :'s are the property names, and the bits to the right are the property values. So, what you have there is a variable called detailInf, that has two properties, hTitle and hMark. hTitle's value is Results, hMark's value is 98.
var detailInf = { "hTitle":"Results", "hMark":"98"};
alert(detailInf.hTitle); //should alert "Results"
alert(detailInf.hMark); //should alert "98
Edit Paolo's answer is better :-)
As Dan F says, that is an object in JSON format. To loop through all the properties of an object you can do:
for (var i in foo) {
alert('foo[' + i + ']: ' + foo[i]);
}