I needed to join a function’s arguments using array.join() and I remembered that I could not simply call join() on the arguments object, as it is not a real array.

Internet wisdom has it that arguments can be changed into an Array by this code snippet:

const args = Array.prototype.slice.call (arguments);

Slicing can cost much more than expected

It came as a surprise to me, when I found this warning in the MDN documentation for arguments:

The JavaScript interpreter in modern browsers is doing lots of optimizations, which are turned off if arguments is leaked outside of a function.

Leaking arguments happens in following cases:

• When the function returns arguments.
• When the function passes arguments to another function (such as call() in the classic Array.prototype.slice.call(arguments)).
• When arguments get captured in a closure.

See Petka Antonov’s explanation.

Workaround

The solution is to create an array with the expected size, and then copy one item after the other.

Here is a working example:

const args = new Array (arguments.length);
for (var i = 0; i < args.length; ++i) {
  args[i] = arguments[i];
}

and obviously, you should not extract this piece of code when doing some refactoring, as you’d end up with leaking again, defeating the purpose of this code snippet.

EDIT: see this follow-up post for the idiomatic ES2015 way of writing equivalent code.