ES6箭头函数的语法以及何时不该使用

ES5:
function f (a, b) {
return a + b;
}
-->ES6
let f = (a, b) => a + b
ES5:
function f (a, b) {
console.log(Hello ES6.);
return a + b;
}
-->ES6
let f = (a, b) => {
console.log(Hello ES6.);
return a + b;
}
如果把函数写码块，return需要自己写returnES5:
function f (a, b) {
return a + b;
}
-->ES6
let f = (a, b) => a + b
ES5:
function f (a, b) {
console.log(Hello ES6.);
return a + b;
}
-->ES6
let f = (a, b) => {
console.log(Hello ES6.);
return a + b;
}
如函在代里面，有return自己写return