Whether javascript allow convert an existed variable to be a constant variable?

You cannot change your var into a const, but assuming that myVar is defined on the global object you can make the binding final using Object.defineProperty:

var myVar = 'sacred-value'

Object.defineProperty(window, 'myVar', {
  writable: false,
  value: window.myVar
})

myVar = 'evil-value'

console.log(myVar) //=> 'sacred-value'

Note: I have assumed the window object is your global context here, but if you are executing in a non-browser environment you may need to use global instead.

You can't change a var into a const.

You could store your variable in an object, mutate it, and then freeze the object with Object.freeze to prevent further mutations:

const variables = {
  a: 'b'
};

variables.a = 'c';
Object.freeze(variables);
variables.a = 'd'; // does nothing
console.log(variables.a);

You cannot change the type of a variable declaration afterwards. However, you can introduce a new scope where you declare it as a constant:

var myVar = "xxx"; // assuming global scope

{
    const myVar = window.myVar; // or `this.` or `global.` depending on your environment
    … // your code here that will throw on assigning to `myVar`
}