Fast alternative to Object.keys(obj).length

Question

How to get number of properties in an Object without the performance penalty that is associated with Object.keys().length?

Answer 1

You could do a for-in loop, it's pretty fast. Basically it iterates through all the keys in the object and increments the counter for each one. Based on a quick benchmark in my console, it's about 5 times faster than Object.keys().length

var counter = 0;
for(var i in obj){
  counter++;
}

Answer 2

You can switch to Map, which is similar to Object, but also you can get size of it, and iterate through keys. It is supported by many browsers, and for older ones there are polyfills.

Answer 3

If you want a fast approach, basically you have two possibilities:

• Keep track of the size of your object manually. Store the size in a variable, increase it when creating a property, and decrease it when deleting.

  var myObj = {},
      size = 0;
  myObj.foo = 1; ++size;
  myObj.bar = 2; ++size;
  size; // 2
  

• Use a similar data structure which exposes the size. For example, ES6 maps.

  var myMap = new Map();
  myMap.set("foo", 1);
  myMap.set("bar", 2);
  myMap.size; // 2