Similar to exporting a module, there are two ways to import a module by using the import keyword. If you want to avoid naming collisions, you can change the name of export both during export as well as import. A Singleton is an object which can only be instantiated only once. If an instance exists, it simply returns a reference to that object.
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So there is only a single object in the memory. Singleton pattern can be implemented using the constructor function. When this constructor function is called, it checks if the instance object exists or not.
And if the object exists, it just returns that object. Singletons can also be implemented using the module pattern. In the above code, we are creating a new instance by calling the singleton. Factory Pattern is a pattern that uses factory methods to create objects without specifying the exact class or constructor function from which the object will be created.click here
The factory pattern is used to create objects without exposing the instantiation logic. This pattern can be used when we need to generate a different object depending upon a specific condition. Here I have created a Car and a Truck class with some default values which is used to create new car and truck objects.
However, it also has its drawbacks. When projects start to grow and we need to start doing more complex code, it becomes harder to implement generic solutions on our programs, making them harder to read and maintain. This is usually known as "Callback Hell".
JS: The Right Way
However, the worst that we can have with callbacks is the inversion of control. If this happens, we are giving control of the program flow sequence to other parties, making it difficult or even impossible!
Promises are harder to master, but address the inversion of control issue. They are a little slower than callbacks, but in return we get a lot of trustability. Promises are a trustable mechanism that also help expressing async code in a more sequential way. They can have, at most, one resolution value, meaning that a Promise always needs to be resolved or rejected. This is how they solve the inversion of control.
Not by removing callbacks, but by creating a mechanism on the wrapper that handles this issue. Promises provide more functionality, like, for example, the Promise. With this kind of operations we can start multiple async requests and proceed only when all of them are resolved or when the first finishes. Also, we can chain multiple Promises on our code without forcing a new level of indentation after each one, using.
This improves the readability of the code, and the maintainability of the program as well, but not everything is perfect. Since this feature is at framework level, there are multiple implementations that can vary on behavior, plus the overhead cost of time and memory. Generators were introduced on ECMAScript and are functions in which we can use and control the iterator, meaning that functions can be paused and resumed at any time. This is a powerful tool for when we want to get each value only when we need, instead of getting all of them at once. We can see in this example that for each next we receive an object with the value and a flag indicating if the generator functions ended.
But generators can be used to control async flows in conjugations with other libraries as well, like in co or redux-saga , of which I will talk more about further ahead.