CODEWITHSUNDEEP
ecmascript-6babeljs
Nick
Nick

Reputation: 5198

Why is babel transforming default arguments this way?

Babel is transpiling my ES2015 default arguments to ES5, but it seems to be doing it very verbosely. Here's the original function:

function initStyles(skipScaling = false) {/*...*/}

And here is the result after transpiling:

function initStyles() {
  var skipScaling = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : false;
}

If I were to (perhaps naively) implement this, I would do something like this:

function initStyles(skipScaling) {
  if (skipScaling === undefined) skipScaling = false;
}

What's the reason Babel handles it this way?

Upvotes: 4

Views: 433

Answers (2)

nonopolarity
nonopolarity

Reputation: 151126

To add some illustration to what @Bergi mentioned it is related to "the number of arguments expected by the function" Function.prototype.length.

(It was arity in the past but now obsolete).

In an ES6 environment,

function foo(s = 123) { 
  console.log(arguments, s) 
}

console.log("foo.length", foo.length);

would print out 0 at the end.

The way Babel or Traceur translate it:

function foo() {
  var s = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : 123;
  console.log(arguments, s);
}

console.log("foo.length", foo.length);

it would also print out 0 at the end.

If written this way:

function bar(s) {
  if (s === undefined) s = 123;
  console.log(arguments, s);
}

console.log("bar.length", bar.length);

Then it'd print out 1 at the end so it is not entirely the same as ES6.

Upvotes: -1

Bergi
Bergi

Reputation: 664969

Parameters that have a default initialiser are not counted for the function arity. The .length of your initStyles function should be 0, and Babel replicates that by using the arguments object to initialise a var instead of using a named parameter.

Upvotes: 7

Related Questions