Javascript array valueOf method - Stack Overflow

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[].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

[].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

• javascript

Share Improve this question Follow edited Jun 22, 2013 at 7:30 Maizere Pathak.Nepal asked Jun 22, 2013 at 3:42 Maizere Pathak.NepalMaizere Pathak.Nepal 2,41144 gold badges3030 silver badges4141 bronze badges 2

• I think it uses toString() not valueOf() – Arun P Johny Commented Jun 22, 2013 at 3:46
• your edit's sample returns a,b, and no, the latter returns a,b because the [].toString() casts all its items to string before concatenating - nested arrays will be stringified recursively. – Fabrício Matté Commented Jun 22, 2013 at 4:05

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3 Answers 3

Sorted by: Reset to default 4

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 mas, 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"

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

write get unlimited arguments separated by mas 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

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 ma anymore.

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