Arrow function改写

代码一：

var materials = [

  'Hydrogen',

  'Helium',

  'Lithium',

  'Beryllium'

];

console.log(materials.map(material => material.length));

// expected output: Array [8, 6, 7, 9]

不用=>改写代码；

代码二：

var materials = [

  'Hydrogen',

  'Helium',

  'Lithium',

  'Beryllium'

];

console.log(materials.map(function(materials) {

          return materials.length;

}));