How to choose what argument a variable is assigned to in a function? -Javascript

Question

Given a normal function as such;

function descriptions(awesome, cool, alright){
   return (awesome || "no one") + " is awesome. " + cool + " is cool. " +
     + alright + " is alright";
}
descriptions("jane", "jack", "jefferson");
//returns "jane is awesome. jack is cool. jefferson is alright."

I would like to use the same function, but would only like to pass it the final two arguments like so:

descriptions(cool : "john", alright : "jane"); //I would like a statement similar to this that works.
//should return "no one is awesome. jack is cool. jefferson is alright."

How would the above be done?

Answer 1

Something different syntactically but similar semantically might be achieved using object destructuring

function descriptions({ awesome = 'no one', cool, alright }) {
    return awesome + " is awesome. " + cool + " is cool. " +
     + alright + " is alright";
}

Then you just invoke it with an object with corresponding properties:

descriptions({ cool: 'a', alright: 'b'});

Answer 2

That is not possible in any variety of ECMAScript (including JavaScript).

It is theoretically possible to do things like use conditional, custom logic:

function(a,b,c){
   if(arguments.length === 1) {
      // we're in object mode;
      b = a.b
      c = a.c
      a = a.a || 'default';
   }
}

But that is not a built in part of the language.

This is NOT possible, for example:

function foo(a,b,c){return a/(b || 1) + c;}
foo({c:1,b:2,a:3})

There is also the possibility to conditionally define values based on number of arguments:

function say (a,b,c) {
   if(arguments.length === 2) {
      c = b;
      b = a;
      a = 'cat';
   }
   console.log('a ' + a + ' likes a ' + b + ' and a ' + c)
}
say('dog', 'bone', 'walk') // a dog likes a bone and a walk
say('mouse', 'bowl of milk') // a cat likes a mouse and a bowl of milk

Answer 3

Yes you can certainly achieve this! You can use a clever trick many developers use to set a variable to a default value if one is not provided.

function descriptions(awesome, cool, alright){
awesome = awesome || "";
if (awesome === "")
{
    return "no one" + " is awesome. " + cool + " is cool. " +
 + alright + " is alright";
}
else{
    return awesome + " is awesome. " + cool + " is cool. " +
 + alright + " is alright";
}

}
console.log(descriptions(undefined, "jack", "jefferson"));

Here is the working code. You could also pass an empty string.

Answer 4

In ECMAScript 6, this can sort of be done if you change your parameters to receive an object and take advantage of destructuring assignment .

 function descriptions({awesome: awesome = "no one", cool: cool = "", alright: alright = ""} = {}) { return awesome + " is awesome. " + cool + " is cool. " + alright + " is alright"; } var res = descriptions({ cool: "john", alright: "jane" }); document.body.textContent = res; 

So we have someone of an emulation of named parameters. Only thing extra needed by the caller is the curly braces.

Of course browser support is limited, but transpilers are available.

Answer 5

You can do this by passing an object:

function descriptions(info) {
    // Avoid TypeError if no argument is passed
    if (!info) {
        info = {};
    }

    return (info.awesome || "no one") + " is awesome. " + (info.cool || "no one") + " is cool. " + (info.alright || "no one") + " is alright.";
}

// Use:
console.log(descriptions({
    awesome: "Strong Bad",
    cool: "The Cheat",
    alright: "Strong Sad"
}));

Answer 6

You could use a different approach:

var coolLevels = {
  isCool: ["Jack", "John"]
, isAlright: ["Jane", "Jefferson"]
, isAwesome: []
}

function describe(people, coolLevel, phrase) {
  return people.filter(function(person){
    return Boolean(coolLevel.indexOf(person))
  }).join(", ") + phrase
}

function descriptions(people){
  var awesome = describe(people, coolLevels.isAwesome, ' is awesome.')
  var cool = describe(people, coolLevels.isCool, ' is cool.')
  var alright = describe(people, coolLevels.isCool, ' is alright.')

  return awesome + cool + alright
}

demo: https://jsbin.com/kahawuhelu/edit?js,console,output

Answer 7

You can pass undefined , null or "" as the first parameter. Eg:

descriptions(null, "jack", "jefferson");

Since you already use awesome || "no one" awesome || "no one" , any falsy value will be good enough.

---

Another approach would be changing the function to receive an object:

function descriptions(options) {
  return (options.awesome || "no one") + " is awesome. " + options.cool + " is cool. " + 
    options.alright + " is alright";
}

descriptions({ cool: "jack", alright: "jefferson" });

---

Now, depending on your browser support, you can use ES6 destructuring parameters:

const descriptions = ({ awesome = 'no one', cool, alright }) => (
  `${awesome} is awesome. ${cool} is cool. ${alright} is alright`
);

descriptions({ cool: 'jack', alright: 'jefferson' });