Object.length undefined in javascript [duplicate]

Question

I have an javascript object of arrays like,

var coordinates = {      "a": [          [1, 2],          [8, 9],          [3, 5],          [6, 1]      ],           "b": [          [5, 8],          [2, 4],          [6, 8],          [1, 9]      ]  }; 

but coordinates.length returns undefined. Fiddle is here.

Answer 1

That's because coordinates is Object not Array, use for..in

var coordinates = {      "a": [          [1, 2],          [8, 9],          [3, 5],          [6, 1]      ],       "b": [          [5, 8],          [2, 4],          [6, 8],          [1, 9]      ]  };  for (var i in coordinates) {   console.log(coordinates[i])  } 

or Object.keys

var coordinates = {   "a": [     [1, 2],     [8, 9],     [3, 5],     [6, 1]   ],    "b": [     [5, 8],     [2, 4],     [6, 8],     [1, 9]   ] };  var keys = Object.keys(coordinates);  for (var i = 0, len = keys.length; i < len; i++) {   console.log(coordinates[keys[i]]); } 

Answer 2

coordinates is an object. Objects in javascript do not, by default, have a length property. Some objects have a length property:

"a string - length is the number of characters".length  ['an array', 'length is the number of elements'].length  (function(a, b) { "a function - length is the number of parameters" }).length 

You are probably trying to find the number of keys in your object, which can be done via Object.keys():

var keyCount = Object.keys(coordinates).length; 

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Be careful, as a length property can be added to any object:

var confusingObject = { length: 100 };