Javascript – Function that modifies an argument, should I return the modified object

javascriptout-parameters

We have a function that modifies a JS object, by adding some custom properties to it. The function doesn't return antyhing

addTransaction: function (obj) {
     obj.transactionId = this.getTransactionId;
     obj.id = this.recordId;
},

Somebody said they preferred that addTransaction return the obj.

Here's what I thought

If I don't return anything (and document that the object is going to be modified), it's kind of clear that the object is going to be modified, as if the name were addTransactionToObj
If I do want to add a return value, I shouldn't modify the incoming object, I should clone the given object, add my properties to the clone and return the clone.
Having a return value that just returns one of the (modified) parameter just sounds wrong

Does anybody have a preference in this matter?

Best Answer

It allows you to do method chaining, which a lot of people feel improves readability. It's a very common idiom in JavaScript, which means a lot of people expect it, especially if the rest of the code base is similar. Your second point about cloning the object also has merit, if used to make your object immutable. How beneficial that is depends on your specific application.

Related Solutions

JavaScript – How to Return Multiple Values in JavaScript

Doing this is the best (and verbose) option right now.

return {x: x, y: y}

Or, less verbose but cleaner

return {x, y}

The caller would have to do this to use the values

var c = get_coords();
alert(c.x + ' ' + c.y);    // c.x and c.y holds the coordinates

If you care to use only single axis you could do -

alert(get_coords().x);

With the ECMAScript 6 you would be able to do destructuring (in caller).

var { x, y } = get_coords();

Too bad that we'll have to wait quite long to use ECMAScript 6 safely in production (in a cross-browser manner).

More info on destructuring - http://fitzgeraldnick.com/weblog/50/

JavaScript Programming Practices – Is a JS Boolean Having Custom Properties a Bad Practice?

In addition to the general design principles, like single responsibility, and least surprise, there's a JavaScript-specific reason that it's not a good idea: there's a huge difference between a boolean and Boolean in JavaScript that prevents it from working in the general case.

boolean is a primitive type, not an object, and cannot have custom properties. Expressions like true.toString() work because behind the scenes, it turns into (new Boolean(true)).toString().

Boolean (with a capital B) is an object, but has very few good uses, and being used as a boolean is definitely not one of them. The reason for that is, that every Boolean is "true", regardless of its value, because all objects get converted to true in a boolean context. For example, try this:

var answer = new Boolean(false);
if (answer) {
  console.log("That was unexpected.");
}

So, in general, there's no way to add properties to a boolean in JavaScript that still lets it behave in a logical way. Modernizr can get away with it because the only add properties to "true" values, which sort of work how you would expect (i.e. they work in if statements). If video isn't supported at all, Modernizr.video will be an actual boolean (with the value false), and can't have properties added to it.

Best Answer

Related Solutions

JavaScript – How to Return Multiple Values in JavaScript

JavaScript Programming Practices – Is a JS Boolean Having Custom Properties a Bad Practice?

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