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.
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.
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.
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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