访问 javascript object 中的计算属性

Question

Why does the console print undefined when I access age property directly. I have to print calcAge function first in order to get the value.

const Osama = {
    firstName: 'osama',
    lastName: 'Shaikh',
    birthyear: 1999,
    job: 'developer',
    friends: ['michael', 'peter', 'steven'],
    haslicense: true,
    calcAge: function () {
        this.age = 2021 - this.birthyear;
        return this.age;
    },

};
console.log(Osama.age);

Answer 1

That object doesn't have an age property to begin with.

You only assign it when you run the calcAge method.

Answer 2

It seems like you're looking to assign a getter for the age property.

 const Osama = { birthyear: 1999, get age() { console.log('in getter'); return 2021 - this.birthyear; }, }; console.log(Osama.age); // in getter, 22 console.log(Osama.age); // in getter, 22

Or, to calculate the age only on first access and replace it with a static property you can use a self-overwriting getter.

 const Osama = { birthyear: 1999, get age() { console.log('in getter'); delete this.age; return (this.age = 2021 - this.birthyear); }, }; console.log(Osama.age); // in getter, 22 console.log(Osama.age); // 22