Javascript array valueOf method

Question

[].valueOf() method retuns array itself.According to this

document.write([["a"]],["b"]) 

should return ['a']b isn't it?But this is not happening ,it just writes ab.I just wanted to know the reason behind this.

For string elements .toString() method returns this,

["a","b"].toString()//a,b

But for elements with array it should return

[["a"],"b"].toString()//[a],b

Answer 1

When you pass an object to document.write, Javascript converts the object to a string with .toString(). In this case, Array.toString() will flatten and join the array with commas, and return it as a string.

["this", "is", "an", "array!"].toString(); // "this,is,an,array!"
[["a",["b"]], ["c"]].toString() // "a,b,c"

We can expand document.write([["a",["b"]], ["c"]]) into the following:

var input = [["a",["b"]], ["c"], "d"];
Array.prototype.verboseToString = function verboseToString() {
  // Make a copy of the array, so we don't destroy the original
  var copy = this.slice(), i;
  for (i = 0; i < copy.length; i++) {
    // If this is an Array, call verboseToString() on it, and go deeper
    if (copy[i] instanceof Array === true) {
      copy[i] = copy[i].verboseToString();
    }
  }
  // copy contains non-arrays and we're ignoring other types' toString() output
  return copy.join(',');
}
document.write(input.verboseToString()); // "a,b,c,d"

Answer 2

document.write([["a"]]",",["b"]) 

write get unlimited arguments separated by commas so it is actually expected behavior

In order to print what you want use:

document.write(["a","b"]) 

This way you will print an array and not a list of arrays

Answer 3

From docs

The text you write is parsed into the document's structure model.

So you send out an array it will just evaluate the array values to string to create a document structure which [["a"]],["b"] has none but just the text values.

If you do this:

document.write(["<a>a</a>", "<a>b</a>"])

You can see it creates 2 anchor elements separated by , so its just array.join(',')

or just provide this:

document.write(["<a>a</a>"], ["<a>b</a>"])

This time it will create 2 anchors you don't see a comma anymore.