The differences and characteristics of let, var and const: What do they mean?

The differences and characteristics of let, var and const: What do they mean?

In JavaScript, let, var and const are keywords used to declare variables. Each of them has different differences and characteristics.

1. let:
   The let keyword was introduced in ES6, which allows us to declare a block-level variable. Block-level scope means that the variable is only visible in the block where it is declared and will not be promoted to the function scope.
   Sample code:

function exampleFunction() {
   if (true) {
      let x = 10;
      console.log(x); // 输出 10
   }
   console.log(x); // 报错，x未定义
}

In the above example, the x variable is declared inside the if block, so it can only be accessed inside the if block.

2. var: The
   var keyword is a keyword used to declare variables in ES5 and previous versions. It has the characteristics of function scope, that is, the variable will be promoted to the function that contains it. top of the domain.
   Sample code:

function exampleFunction() {
   if (true) {
      var x = 10;
      console.log(x); // 输出 10
   }
   console.log(x); // 输出 10
}

In the above example, the x variable is declared inside the if block, but because var has the characteristics of function scope, it can also be accessed outside the if block.

3. const: The
   const keyword is also introduced in ES6. It is used to declare constants. Once assigned, they cannot be modified. At the same time, const also has the characteristics of block-level scope.
   Sample code:

function exampleFunction() {
   if (true) {
      const x = 10;
      console.log(x); // 输出 10
   }
   console.log(x); // 报错，x未定义
}

In the above example, x is declared as a constant and cannot be modified.

It should be noted that constants declared as const are immutable, but if the constant is an object or array, their attributes or elements can be modified.
Sample code:

const obj = {
   name: 'Alice'
};
obj.name = 'Bob'; // 可以修改obj的属性

const arr = [1, 2, 3];
arr.push(4); // 可以修改arr的元素

To sum up, let is used to declare block-level scope variables, var is used to declare function scope variables, and const is used to declare constants. When using, we should choose appropriate keywords according to needs.

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