javascript - 仅使用 JavaScript 代理将偶数添加到数组

Question

我正在学习 JavaScript 代理。
通过使用 设置陷阱我只想将偶数添加到数组中。但每次抛出

Uncaught TypeError: 'set' on proxy: trap returned falsish for property'length'

这是我的代码示例。

//Only even numbers will be added here. We are using Js Proxy

let evenNumbers = [];

evenNumbers = new Proxy(evenNumbers, {
    set(target, p, value, receiver) {
        if ( (value % 2) === 0 ) {
            target[p] = value;
            return true
        }else {
            return false;
        }

    }
});

evenNumbers.push(2);

Answer 1

您需要返回 true对于这两种情况，因为

Return value

The set() method should return a boolean value.

• Return true to indicate that assignment succeeded.
• If the set() method returns false, and the assignment happened in strict-mode code, a TypeError will be thrown.

push调用代理两次，一次是通过推送值，另一次是更改 length属性(property)。为避免在此代理中进行不必要的更多操作，您可以使用此不需要的属性提前退出。

let evenNumbers = [];

evenNumbers = new Proxy(evenNumbers, {
    set (target, p, value, receiver) {
        if (p === 'length') return true; // exclude this property
        if (value % 2 === 0) {
            target[p] = value;
        }
        return true;
    }
});

evenNumbers.push(2);
console.log(evenNumbers.length); // 1
evenNumbers.push(3);
console.log(evenNumbers.length); // Still 1
console.log(evenNumbers);        // [ 2 ]