Sets

ECMAScript 6 introduces sets. Sets are collections of values and can be iterated in the order of the insertion of their elements. An important characteristic about sets is that a value can occur only once in a set.

The following snippet shows some basic operations on sets:

var mySet = new Set();
mySet.add(1);
mySet.add("Howdy");
mySet.add("foo");

mySet.has(1); // true
mySet.delete("foo");
mySet.size; // 2

for (let item of mySet) console.log(item);
// 1
// "Howdy"

We discussed briefly that JavaScript arrays are not really arrays in a traditional sense. In JavaScript, arrays are objects that have the following characteristics:

• The length property
• The functions that inherit from Array.prototype (we will discuss this in the next chapter)
• Special handling for keys that are numeric keys

When we write an array index as numbers, they get converted to strings—arr[0] internally becomes arr["0"]. Due to this, there are a few things that we need to be aware of when we use JavaScript arrays:

• Accessing array elements by an index is not a constant time operation as it is in, say, C. As arrays are actually key-value maps, the access will depend on the layout of the map and other factors (collisions and others).
• JavaScript arrays are sparse (most of the elements have the default value), which means that the array can have gaps in it. To understand this, look at the following snippet:

  var testArr=new Array(3);
  console.log(testArr); 

  You will see the output as [undefined, undefined, undefined]—undefined is the default value stored on the array element.

Consider the following example:

var testArr=[];
testArr[3] = 10;
testArr[10] = 3;
console.log(testArr);
// [undefined, undefined, undefined, 10, undefined, undefined, undefined, undefined, undefined, undefined, 3]

You can see that there are gaps in this array. Only two elements have elements and the rest are gaps with the default value. Knowing this helps you in a couple of things. Using the for...in loop to iterate an array can result in unexpected results. Consider the following example:

var a = [];
a[5] = 5;
for (var i=0; i<a.length; i++) {
  console.log(a[i]);
}
// Iterates over numeric indexes from 0 to 5
// [undefined,undefined,undefined,undefined,undefined,5]

for (var x in a) {
  console.log(x);
}
// Shows only the explicitly set index of "5", and ignores 0-4