JavaScript Collections

Basic arrays

• Collection of objects of any type

  • Can be a mixed type

    • Numbers, Strings, Objects, Functions

• Element access

• Length

• Holes

• Iteration

Iteration

• Go over all the elements in a collection

• Concepts:

  • Iterable -> An object whose contents can be accessed sequentially

  • Iterator -> Pointer to the next element

• Iterable Objects:

  • Array

  • String

  • Map

  • Set

  • Browser DOM - tree structure

• Objects

  •   object.keys()
    

  •   object.entries()
    

  • These are some helper functions

Iterations and Transformations

• Functions that take functions as input

•   map, filter, find
    //Apply a callback function over each element of array
  

• Elements of functional programming

  • Create a transformation programming

Callback -> Function passed in to another function to be called for some purpose

Other Collections

• Maps -> Proper dictionaries instead of objects

• WeakMaps

• Sets

More advanced stuff -> use only when needed

Destructuring

• Simple syntax to split an array into multiple variables

• Easier to pass and collect arguments, etc

• Also possible for objects

Generators

• Functions that yield values one at a timed

• Computed iterables

• Dynamically generate iterators

Basic array

let x = [2, 3, 5, 5];
console.log(`${typeof x}: ${x} with length = ${x.length}`);
console.log(x[0]);

Output: object: 2,3,5,5 with length = 4

2

Mixed element array

let y = [1, 'b', a => a + 1];
console.log(`${typeof(y)}: ${y} with length = ${y.length}`);
console.log(x.length, y.length);

Output: 1, b, a => a + 1 with length = 3

4,3

Holes in the array

This creates holes in the array

We have changed the size of the array without adding any element, so the elements are undefined

y.length = 10;
console.log(`${typeof(y)}: ${y} with length = ${y.length}`);