函数作为函数参数和返回值

函数作为另一个函数的参数还是作为返回值，这两种应用情形都体现了在javascript中，函数是一个对象，是一个value.

1. 函数作为返回结果

function createCompareFunction(propertyName) {
    return function compareFunction(object1, object2, propertyName) {
        let value1 = object1[propertyName];
        let value2 = object2[propertyName];
        return value1 < value2 ? -1 :
            value1 > value2 ? 1:
            0;
    };    
}
data = [{name:"Amy",age:20},{name:"Sheldon",age:22},{name:"penny",age:21}];
data.sort(createCompareFunction("age"));
console.log(data);

2. 函数作为另一个函数的函数

function ask(age, yes, no) {
  return age >= 18 ? yes() : no();
} 
let age=prompt("How old are you?","");
ask(age,
  function(){ alert("Welcome to this website!");},
  function(){ alert("Teenagers are not welcomed on the page!");});