javascript - Why there's a total index number of arrays in concat result -

var = ['a', 'b', 'c']; var b = ['d', 'e', 'f']; var c = ['g', 'h', 'i']; var d = [a, b, c]; (e in d) var f = d.concat(e); console.log(f); //[array[3], array[3], array[3], "2"] 

why there's 2 in there? how remove 2, before result come out? (not alter result)

"2" length property of resulting array. included, because loop using for ... in. if use for (var i=0; i<d.length; i++) {...}, not included. see snippet. anyway, seems me simplify whole enchillada using var f = d.slice().

citing mdn:

array indexes enumerable properties integer names , otherwise identical general object properties. there no guarantee for...in return indexes in particular order , return enumerable properties, including non–integer names , inherited.
because order of iteration implementation-dependent, iterating on array may not visit elements in consistent order. therefore better use loop numeric index (or array.prototype.foreach() or for...of loop) when iterating on arrays order of access important.

see ...

var = ['a', 'b', 'c'];  var b = ['d', 'e', 'f'];  var c = ['g', 'h', 'i'];  var d = [a, b, c];  var f = [];  (var i=0; < d.length; i+=1) {     f.push(d[i]);  }  document.queryselector("#result").textcontent = json.stringify(f, null, " ");

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