CODEWITHSUNDEEP
javascriptloopsarray.prototype.map
aayush
aayush

Reputation: 85

Understanding Array.prototype.map

I was going through the Javascript map function, and stuck at the loop syntax. It is difficult to figure out the o placed after comma. Can anybody help me figure it out. Also what is the terminating condition for loop ?

Array.prototype.mymap = function (callback) {
  var obj = Object(this);

  if (obj.length === 0) return null;
  if (typeof(callback) === 'undefined') return null;

  for (var i = 0, o; o = obj[i]; i++) {
    obj[i] = callback(o);
  }

  return obj;
};

Upvotes: 0

Views: 145

Answers (1)

deceze
deceze

Reputation: 522016

for (var i = 0, o; o = obj[i]; i++) {
    obj[i] = callback(o);
}

This is the same as:

var i = 0,
    o;

while (o = obj[i]) {
    ...
    i++;
}

Which means, it declares the variable o, which is initially set to undefined. During each loop iteration, obj[i] is assigned to o. When obj[i] results in undefined (because i is beyond the length of the array), the expression o = obj[i] results in undefined, which terminates the loop.

Actually, this loop implementation has a bug: it terminates whenever any array value is falsey; which is probably not desired.

Upvotes: 6

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