JSFiddle - React, Tailwind, and code Playground

JavaScript

let object = {
  "@context": "https://schema.org",
  "@graph": [   

    {
      "@type": "WebPage",
      "@id": "id234234",
      "url": "https://www.whatevver.com",
      "inLanguage": "de-DE",
      "name": "this is the name",
      "isPartOf": {
        "@id": "id234234" // MAKE IT AS A STRING
      },
      "primaryImageOfPage": {
        "@id": "id234234"
      },
      "datePublished": "2020-01-14T07:46:12+00:00",
      "dateModified": "2020-01-15T08:04:50+00:00",
      "description": "description "
    },

  ]
}

console.log(object)

const getpublishedDate = () => {
  let publishedDate = "";
  const yoastInfo = object
  vars = yoastInfo["@graph"];
  for(var i in vars) {
    if(vars[i]['@type'] === 'WebPage') {
      publishedDate=(vars[i]['datePublished']);
    }
  }
  console.log(publishedDate)
  return publishedDate;
}

getpublishedDate()