数组中可以有Getter / Setter吗？

Question

I've just learnt about Getters/Setters for objects and decided to play with it. However, I tried to put a Getter in an array and failed to do so. Is it not possible or am I just doing something wrong?

const obj = {
  a: 1,
  get b() { return this.a + 1 } 
}

console.log(obj.b) //2

For Array

const arr = [
  1,
  get () { return this[0] + 1 } 
]

console.log(arr[1]) // Doesn't work

Answer 1

Is it not possible or am I just doing something wrong?

It's not possible using an array initializer (loosely, an "array literal").

You could define an accessor property after the fact, though, via Object.defineProperty :

 const arr = [1]; Object.defineProperty(arr, "1", { get() { return this[0] + 1; } }); console.log(arr[1]); // 2 

You can combine that, I've left the steps above separate for clarity, but Object.defineProperty returns the object you call it on, so:

 const arr = Object.defineProperty([1], "1", { get() { return this[0] + 1; } }); console.log(arr[1]); // 2 

I wouldn't , but you can. :-) Why wouldn't I? Because arrays expect their properties with array index names¹ to be simple data properties, not accessors, and so various array methods (like splice ) will fail if they try to set the "1" property to a different value (you could fix that by adding a setter). In general, though, I'd leave array entry properties alone.

That said, if the property has both a getter and a setter, I can't immediately think of a problem with it. Here's the above with a setter, just for completeness:

 const arr = Object.defineProperty([1], "1", { get() { return this[0] + 1; }, set(value) { this[0] = value - 1; } }); console.log(arr); // [1, 2] arr[1] = 42; console.log(arr); // [41, 42] arr[0] = 27; console.log(arr); // [27, 28] 

 .as-console-wrapper { max-height: 100% !important; } 

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¹ "array index names" - Here's what the spec says about property names that are array indexes:

An integer index is a String-valued property key that is a canonical numeric String (see 7.1.16) and whose numeric value is either +0 or a positive integer ≤ 2 ⁵³ - 1. An array index is an integer index whose numeric value i is in the range +0 ≤ i < 2 ³² - 1.

Answer 2

Instead of using a setter/getter, you could use a Proxy instead and evaluate the accessor and return different values.

 var handler = { get: function(obj, prop) { return prop in obj ? obj[prop] : obj[0] + 1; } }; var p = new Proxy([1], handler); console.log(p[1]); p.push(10) console.log(p[1]); console.log(p[2]);