The difference between const, var, let in javascript

Today, when I encounter variables defined by const, I summarize the differences between const, var, and let.

1. Variables defined by const cannot be modified and must be initialized.

const b =4;//正确
 const b;//错误，必须初始化 

console.log('函数外const定义b：' + b);//有输出值
console.log('函数外修改const定义b：' + b);//无法输出 

2. The variable defined by var can be modified. If it is not initialized, it will output undefined and no error will be reported.

3. Let is a block-level scope. After the function is defined with let, it has no effect on the outside of the function.

  let c = 2;
  console.log('函数外let定义c：' + c);//输出c=2
  function change(){
      let c = 3;
      console.log('函数内let定义c：' + c);//输出c=3
  } 
  change();
  console.log('函数调用后let定义c不受函数内部定义影响：' + c);//输出c=2