Temporal Dead Zone is a behaviour that occurs with variables declared using let and const keywords.

It is a behaviour where we try to access a variable before it is initialized.

A temporal dead zone (TDZ) is the area of a block where a variable is inaccessible until the moment the computer completely initializes it with a value.

Object destructuring is a new way to extract elements from an object or an array.

Destructuring is a JavaScript expression that allows us to extract data from arrays, objects, and maps and set them into new, distinct variables. Destructuring allows us to extract multiple properties, or items, from an array at a time.

In javascript, Map is used to store key-value pairs. The key-value pairs can be of both primitive and non-primitive types.

WeakMap is similar to Map with key differences:

The keys and values in weakmap should always be an object.

If there are no references to the object, the object will be garbage collected.

In javascript, Set is a collection of unique and ordered elements.

Just like Set, WeakSet is also a collection of unique and ordered elements with some key differences:

Weakset contains only objects and no other type.

An object inside the weakset is referenced weakly. This means, if the object inside the weakset does not have a reference, it will be garbage collected.

Unlike Set, WeakSet only has three methods, add() , delete() and has() .

Introduced in ES6 version, generator functions are a special class of functions.

They can be stopped midway and then continue from where it had stopped.

Generator functions are declared with the function* keyword instead of the normal function keyword:

In normal functions, we use the return keyword to return a value and as soon as the return statement gets executed, the function execution stops:

In the case of generator functions, when called, they do not execute the code, instead they return a generator object . This generator object handles the execution.

The generator object consists of a method called next() , this method when called, executes the code until the nearest yield statement, and returns the yield value.

As one can see the next method returns an object consisting of value and done properties.

Value property represents the yielded value.

Done property tells us whether the function code is finished or not. (Returns true if finished)

Generator functions are used to return iterators. Let’s see an example where an iterator is returned

Promises are used to handle asynchronous operations in javascript.

Before promises, callbacks were used to handle asynchronous operations. But due to limited functionality of callback, using multiple callbacks to handle asynchronous code can lead to unmanageable code.

Promise object has four states –

Pending – Initial state of promise. This state represents that the promise has neither been fulfilled nor been rejected, it is in the pending state.
Fulfilled – This state represents that the promise has been fulfilled, meaning the async operation is completed.
Rejected – This state represents that the promise has been rejected for some reason, meaning the async operation has failed.
Settled – This state represents that the promise has been either rejected or fulfilled.

A promise is created using the Promise constructor which takes in a callback function with two parameters, resolve and reject respectively.

resolve is a function that will be called, when the async operation has been successfully completed.

reject is a function that will be called, when the async operation fails or if some error occurs.

We can consume any promise by attaching then() and catch() methods to the consumer.

then() method is used to access the result when the promise is fulfilled.

catch() method is used to access the result/error when the promise is rejected.

Spread operator was introduced in the ES6 version of javascript.

Spread operator (…)

Although the syntax of spread operator is exactly the same as the rest parameter, spread operator is used to spread an array, and object literals. We also use spread operators where one or more arguments are expected in a function call.

REST parameter was introduced in the ES6 version of javascript.

Rest parameter ( … )

It provides an improved way of handling parameters of a function.

Using the rest parameter syntax, we can create functions that can take a variable number of arguments.

Any number of arguments will be converted into an array using the rest parameter.

It also helps in extracting all or some parts of the arguments.

Rest parameter can be used by applying three dots (…) before the parameters.