Let’s talk about common function decoration patterns in JavaScript

Function decoration is a design pattern that changes the behavior of a function by adding extra code inside it. In JavaScript, you can use function decoration to perform some operations when a function is called, such as validating input parameters, modifying return values, or caching function calculation results, etc. This article will help you better understand and apply this technology by introducing common function decoration patterns in JavaScript.

1. Decorator pattern

The decorator pattern is a structural design pattern that allows objects to dynamically add new functions or behaviors at runtime. In JavaScript, a function is also an object, so we can use the decorator pattern to decorate functions.

The following example shows how to use the decorator pattern to add input validation functionality:

function validateInput(fn) {
  return function(...args) {
    for(let arg of args) {
      if(typeof arg !== 'number') {
        throw new Error('Invalid input, input should be a number');
      }
    }
    return fn(...args);
  }
}

function addNumbers(a, b) {
  return a + b;
}

const addNumbersWithValidation = validateInput(addNumbers);

console.log(addNumbersWithValidation(1, 2)); //output: 3
console.log(addNumbersWithValidation('1', 2)); //throws Error: Invalid input, input should be a number

In the above example, we define a function named validateInput , which receives a function as a parameter and returns a new function. This new function will first verify whether the input parameter is a number, and if so it will call the original function, otherwise it will throw an error.

Finally, we decorate the addNumbers function with validateInput, create a new function addNumbersWithValidation, and use it to add two numbers . When the input parameters are strings and numbers, we can see that the behavior of addNumbersWithValidation is different from the original addNumbers because it validates the input parameters.

When using the decorator pattern, you need to pay attention to the following points:

• The decorator function should return a new function.
• The new function should accept the same parameters as the original function and perform some additional operations before or after calling the original function.
• Decorator functions can be composed. For example, we could write one decorator for input validation and another for output caching.

2. Higher-order functions

In JavaScript, functions are first-class citizens, which means that we can treat functions like ordinary variables. Higher-order functions are those that receive one or more functions as arguments and return a new function. Using higher-order functions allows us to handle functions more flexibly, with higher readability and reusability.

The following example shows a simple higher-order function:

function higherOrderFunction(a, b, callback) {
  const result = callback(a, b);
  console.log(`The result is ${result}`);
}

function add(a, b) {
  return a + b;
}

higherOrderFunction(1, 2, add); //output: The result is 3

In the above example, we defined a function named higherOrderFunction, which receives two number and a callback function as parameters. Inside the higherOrderFunction function, we call the callback function and output its return value to the console.

Finally, we use the add function to operate and pass it as a callback function to higherOrderFunction. Inside higherOrderFunction, we output the result of the add call to the console.

In short, higher-order functions are a powerful tool that allow us to handle functions and variables more flexibly, and provide higher readability and reusability.

3. Function Currying

Function currying is a technique that is achieved by converting a multi-parameter function into a series of single-parameter functions. This way, we can store a portion of the parameters in each single-argument function and use these partial arguments to call the original function. Function currying can make code more concise and readable, and naturally supports abstract operations such as function composition and function composition.

The following example shows how to use currying to split multiple parameters into multiple steps:

function add(a) {
  return function(b) {
    return function(c) {
      return a + b + c;
    }
  }
}

console.log(add(1)(2)(3)); //output: 6

In the above example, we define a method that receives one parameter and returns A function of functions. Calling the returned function also returns a function. Finally, the innermost function directly adds all parameters together and returns the corresponding result.

When using function currying, you need to pay attention to the following points:

• Each single-parameter function should only receive one parameter.
• Every single-argument function should return another function, or eventually return some value.
• Function currying can be implemented using the bind method. For example, const add10 = add.bind(null, 10) can be used to create a add10 function that passes 10 as the first argument to add function.

4. Conclusion

By using the decorator pattern, higher-order functions and function currying, we can process and combine functions more flexibly, and implement some advanced functions, such as Validate input, cache results, combine functions, and more. These techniques are very common in JavaScript, so they are worth learning and mastering in depth.

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