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JavaScript array value of the method

 3 years ago
source link: https://www.codesd.com/item/javascript-array-value-of-the-method.html
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JavaScript array value of the method

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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

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"
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