为什么在 object 的 function 属性中声明变量之前可以引用它？

Question

Basically, I'm looking for an explanation of why and how this is working

const someObj = {
    test: () => callback()
}

const callback = () => console.log("how?")

someObj.test()

output: how?

and this is not

const someObj = {
    test: callback()
}

const callback = () => console.log("how?")

someObj.test

output: Uncaught ReferenceError: Cannot access 'callback' before initialization

Answer 1

In your first example you are assigning a function to the property test. It doesn't try to call callback until you call it. By the time you do call it callback has been defined. Try calling test before you define callback and you will see if also fails.

In your second example you are trying to assign the result of calling callback to the property test, but seeing callback hasn't been defined yet you get the error.