Understand the basic principles of JavaScript closures

JavaScript closure is a relatively complex concept. Many beginners find it difficult to understand closures, but they are an important concept for understanding JavaScript and writing high-quality code.

What is closure?
Closure means that a function can access variables outside its scope. These variables still exist even outside the function. This is because when a function is defined, it creates a scope of its own that contains all of the function's parameters and variables defined inside the function. When the function returns, the scope still exists, and the function will continue to access the variables in the scope. This is a closure.

Basic principle:
When a function is called, its scope chain will include variables in the current scope and all parent scopes. If the function has nested functions, the scope chain of the nested function will also include variables from the parent scope and the current scope. In such an environment, an inner function can access the variables of the outer function, but the outer function cannot access the variables of the inner function. This mechanism is called lexical scoping or static scoping.

Application of closures:
Closures are widely used in many design patterns and techniques in JavaScript. The following are some practical use cases of using closures:

1. Obtain private variables:
Because there is no concept of private variables in JavaScript, using closures can simulate private variables. As long as a variable is defined inside a function, it will exist in the scope of that function until the nested function returns or is deleted.

For example, the following code example demonstrates how to use a closure to create a counter:

function createCounter() {
let count = 0;
function counter() {

count++;
console.log(count);

}
return counter;
}
const myCounter = createCounter();
myCounter(); //1
myCounter(); //2
myCounter(); //3

2. Delayed execution:
Using closures, you can create a function to implement delayed execution. When you need to wait for a period of time before performing a function, you can use closures to accomplish this.

For example, the following code demonstrates how to use closures to implement a delayed execution function:

function delayedFunction() {
let timer;
return function() {

clearTimeout(timer);
timer = setTimeout(() => {
  console.log('Delayed function executed!');
}, 1000);

}
}
const myDelayedFunction = delayedFunction();
myDelayedFunction(); //Wait for 1 second and output 'Delayed function executed!'

Closure although It may seem difficult, but it's a very useful concept for JavaScript developers. Understanding the basic principles of closures is key to understanding higher-order functions and design patterns in JavaScript. I hope this article helps you better understand closures and apply them to your programs.

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