how to do `var self = this` inside es6 class?

How can we do like var self = this; as we used to do in ES5?

You can do it exactly like you did in ES5 – ES6 is completely backward-compatible after all:

class Point {
    constructor(x) {
        this.x = x;
        var self = this;
        this.toString = function() {
            return self.x;
        };
    }
}

However, that’s really not idiomatic ES6 (not talking about const instead of var). You’d rather use an arrow function that has a lexical-scoped this, so that you can avoid this self variable completely:

class Point {
    constructor(x) {
        this.x = x;
        this.toString = () => {
            return this.x;
        };
    }
}

(which could even be shortened to this.toString = () => this.x;)