Arrow function的restore

var materials = [  

'Hydrogen',    

'Helium',    

'Lithium',  

'Beryllium'

];

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

标准函数形式：

 

var materials = [  

'Hydrogen',    

'Helium',    

'Lithium',  

'Beryllium'

];

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

　　return materils.length()

}));