用JavaScript解析Json-Google跟踪代码管理器

我有那个脚本:

function() {
  var publishedDate = "";
  var yoastInfo = JSON.parse(document.head.querySelector('.yoast-schema-graph.yoast-schema-graph--main').innerHTML);
  vars = yoastInfo["@graph"];
  for(var i in vars) {
    if(vars[i]['@type'] === 'WebPage') {
      pubDateRaw=(vars[i]['datePublished']);
      break;
    }
  }
  publishedDate.concat(pubDateRaw.substring(0,4),pubDateRaw.substring(5,7),pubDateRaw.substring(8,10));
  return publishedDate;
}

When I run the code, it gives me a "" instead of the date.

这是json:

{
  "@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
      },
      "primaryImageOfPage": {
        "@id": "id234234"
      },
      "datePublished": "2020-01-14T07:46:12+00:00",
      "dateModified": "2020-01-15T08:04:50+00:00",
      "description": "description "
    },

  ]
}

所以我需要从该json中提取datePubished。我的代码有什么问题?

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