What element sees the method has for WeakSet?

[carriann]

There's a code:

let messages = [
  { text: "Hello", from: "John" },
  { text: "How goes?", from: "John1" },
  { text: "See you soon", from: "Alice" }
];
let answer = new WeakSet();
answer.add(messages[0]);

answer.add(messages[1]);

answer.add(messages[2]);
messages.shift()
alert("0: " + answer.has(messages[0]));

alert("1: " + answer.has(messages[1]));

alert("2: " + answer.has(messages[2]));

Method shift() removes the first element of the mass, in our case { text: "Hello", from: "John" }The screen shall be displayed:

0: true

1: true

2: false

Why? 0: true?

If the method shift() is to delete messages[0], the conclusion will be correct:

0: false

1: true

2: true

If a splice(0) method is used instead of the shift method, the conclusion will be:

0: true

1: true

2: false

I mean, it's not right either.

[derk]

That's right.

That's how it works. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/shift and https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/splice

The body is removed, the rest are moving.

I mean, after the call.

messages.shift()

or

messages.splice(0, 1)

♪ messages The following elements will remain:

[
  { text: "How goes?", from: "John1" },
  { text: "See you soon", from: "Alice" }
];

So you're actually checking on the first and second element and under index 2 is lying. undefined - because there's no element with such an index in the mass.