Objects

We use objects extremely frequently — probably more than any other data structure. In this section, we'll cover:

• Adding and removing keys
• Checking existence of keys
• Clone and merging
• Converting to and from arrays

Generally we'll only use these techniques with "plain objects", but these all technically work on class instances too.

Adding and removing elements

We add new elements or update existing elements by assigning a value to a specific key. We remove elements with the delete operator.

Using delete is different than assigning an undefined value! Deleting removes the key altogether.

Checking existence

We use the in operator and the hasOwnProperty method to check if an object contains a key.

The difference: in also returns true for inherited properties.

Cloning and merging

We use Object.assign or spread syntax ... to clone and merge objects.

If keys conflict, the last object merged takes precedence.

Just like with arrays, merging is shallow; we only copy references to elements.

Creating arrays from objects

We often want to run a function for each key, each value, or each key-value pair in an object. To do this, We generally convert the object into an array, and then transform it using array methods.

We can use the static methods Object.keys, Object.values, and Object.entries to transform an object into an array.

While we can use a for ... in loop, it's much less common. We mainly use for ... in for performance reasons or if we want to exit the loop early, but the difference is small.

Creating objects from arrays

After transforming an object to an array, we often want to transform the array back to an object.

If we used Object.entries, we can then use Object.fromEntries. Alternately, we can achieve more custom behavior with reduce.