let、var 或 const 之间有什么区别？

使用 var 关键字声明的变量的作用域为创建它们的函数，或者如果在任何函数外部创建，则为全局对象。 let 和 const 是块作用域的，这意味着它们只能在最近的一组花括号（函数、if-else 块或 for 循环）内访问。function foo() { // all variables are accessible within functions. var bar = 'bar'; let baz = 'baz'; const qux = 'qux'; console.log(bar); // bar console.log(baz); // baz console.log(qux); // qux}console.log(bar); // referenceerror: bar is not definedconsole.log(baz); // referenceerror: baz is not definedconsole.log(qux); // referenceerror: qux is not definedif (true) { var bar = 'bar'; let baz = 'baz'; const qux = 'qux';}// var declared variables are accessible anywhere in the function scope.console.log(bar); // bar// let and const defined variables are not accessible outside the block they were defined in.console.log(baz); // referenceerror: baz is not definedconsole.log(qux); // referenceerror: qux is not defined登录后复制var 允许变量被提升，这意味着它们可以在声明之前在代码中引用。 let 和 const 不会允许这样做，而是会抛出错误。console.log(foo); // undefinedvar foo = 'foo';console.log(baz); // referenceerror: can't access lexical declaration 'baz' before initializationlet baz = 'baz';console.log(bar); // referenceerror: can't access lexical declaration 'bar' before initializationconst bar = 'bar';登录后复制用var重新声明变量不会报错，但let和const会报错var foo = 'foo';var foo = 'bar';console.log(foo); // "bar"let baz = 'baz';let baz = 'qux'; // uncaught syntaxerror: identifier 'baz' has already been declared登录后复制let 和 const 的区别在于 let 允许重新分配变量的值，而 const 则不允许。// This is fine.let foo = 'foo';foo = 'bar';// This causes an exception.const baz = 'baz';baz = 'qux';登录后复制 以上就是let、var 或 const 之间有什么区别？的详细内容，更多请关注我的其它相关文章！