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Showing content from https://developer.mozilla.org/pt-BR/docs/Web/JavaScript/Reference/Global_Objects/WeakMap/has below:

WeakMap.prototype.has() - JavaScript | MDN

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O método has() retorna um booleano indicando se um elemento com a chave especificada existe no objeto WeakMap ou não.

const weakmap1 = new WeakMap();
const object1 = {};
const object2 = {};

weakmap1.set(object1, "foo");

console.log(weakmap1.has(object1));
// Expected output: true

console.log(weakmap1.has(object2));
// Expected output: false

chave

Obrigatório. A chave do elemento cuja presença deve ser verificada no objeto WeakMap.

Boolean

Retorna true se um elemento com a chave especificada existir no objeto WeakMap. Caso contrário, retorna false.

var wm = new WeakMap();
wm.set(window, "foo");

wm.has(window); // retorna true
wm.has("baz"); // retorna false

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