Nao entendo pq nao esta funcionando

Stack.es6

/**
 * Write by wdiasvargas on 04/05/17.
 *   www.williamdiasvargas.com.br
 *
 **/
'use strict'
//STACK com ES6
import push from './stack_methods/push'
import pop from './stack_methods/pop'
import peek from './stack_methods/peek'
import isEmpty from './stack_methods/isEmpty'
import clear from './stack_methods/clear'
import size from './stack_methods/size'
import print from './stack_methods/print'

class Stack {
  constructor () {
    this.data = [];
  }
  push = push;
  pop = pop;
  peek = peek;
  isEmpty = isEmpty;
  clear = clear;
  size = size;
  print = print;

}

let stack = new Stack();
console.log(stack.print())


// isEmpty.es6 export default () => this.data.length == 0;
// size.es6 export default () => this.data.length;
// push.es6 export default (element) => this.data.push(element)
// pop.es6 export default () => this.data.pop() || -1;
// peek.es6 export default () => this.data[this.data.length -1] || -1;
// clear.es6 export default() => this.data = [];
// print.es6 export default () => console.log(this.data.toString())