Let、Const 和 Var 概述：主要差异解释

function varexample() {    if (true) {        var x = 10; // x is function-scoped    }    console.log(x); // outputs: 10}varexample();if (true) {    var y = 20; // y is globally scoped because it's outside a function}console.log(y); // outputs: 20

function letexample() {    if (true) {        let x = 10; // x is block-scoped        console.log(x); // outputs: 10    }    console.log(x); // referenceerror: x is not defined}letexample();if (true) {    let y = 20; // y is block-scoped    console.log(y); // outputs: 20}console.log(y); // referenceerror: y is not defined

function constexample() {    if (true) {        const x = 10; // x is block-scoped        console.log(x); // outputs: 10    }    console.log(x); // referenceerror: x is not defined}constexample();if (true) {    const y = 20; // y is block-scoped    console.log(y); // outputs: 20}console.log(y); // referenceerror: y is not defined

console.log(myvar); // outputs: undefinedvar myvar = 10;

console.log(mylet);// referenceerror: cannot access 'mylet' before initializationlet mylet = 10;

console.log(myconst); // referenceerror: cannot access 'myconst' before initializationconst myconst = 20;

var x = 10;x = 20; // reassignmentconsole.log(x); // outputs: 20var x = 30; // reinitializationconsole.log(x); // outputs: 30

let y = 10;y = 20; // reassignmentconsole.log(y); // outputs: 20let y = 30; // syntaxerror: identifier 'y' has already been declared

const z = 10;z = 20; // typeerror: assignment to constant variable.const z = 30; // syntaxerror: identifier 'z' has already been declared

const obj = { a: 1 };obj.a = 2; // allowed, modifies the propertyconsole.log(obj.a); // outputs: 2obj = { a: 3 }; // typeerror: assignment to constant variable.

const arr = [1, 2, 3];arr[0] = 4; // Allowed, modifies the elementconsole.log(arr); // Outputs: [4, 2, 3]arr = [5, 6, 7]; // TypeError: Assignment to constant variable.