How to get the last item in a javascript-value object with a for loop?

Question

var obj = { \'a\' : \'apple\', \'b\' : \'banana\', \'c\' : \'carrot\' }

If I do a

for(key in obj) {
  console.log( key + \' has a value         


        

Answer 1

don't use for (key in obj), it will iterate over all enumerable properties including prototype properties, and can lead to amazingly horrible things. Modern JS has a special function for getting only the relevant keys out of an object, using Object.keys(...), so if you use var keys = Object.keys(obj) to get the list of keys as an array, you can then iterate over that:

// blind iteration
Object.keys(obj).forEach(function(key, i) {
  var value = obj[key];
  // do what you need to here, with index i as position information.
  // Note that you cannot break out of this iteration, although you
  // can of course use ".some()" rather than ".forEach()" for that.
});

// indexed iteration
for(var keys = Object.keys(obj), i = 0, end = keys.length; i < end; i++) {
  var key = keys[i], value = obj[key];
  // do what you need to here, with index i as position information,
  // using "break" if you need to cut the iteration short.
});

or select its last element immediately

var keys = Object.keys(obj);
var last = keys[keys.length-1];

or using a slice:

var keys = Object.keys(obj);
var last = keys.slice(-1)[0];

or using a shift (but that's a destructive operation, so we're not caching the keys because the shift turns it into "not all the keys anymore"):

var last = Object.keys(obj).shift();