let line = "xyz";

let data = [
    {
        "header": {
            "po_no": "P.O. Number"
        },
        "line": line
    },
    {
        "header": {
            "po_no": "Another P.O. Number"
        },
        "line": line
    }
];

data.filter(item => {
  return item.header.po_no === 'P.O. Number' // Evaluates to true
})

[{
  "header": {
    "po_no": "P.O. Number"
  },
  "line": line
}]

const expectedItems = data.filter(item => {
  return item.header.po_no === 'P.O. Number' // Evaluates to true
});

let line = "xyz";

let data = [
  {
    "header": {
    "po_no": "P.O. Number"
    },
    "line": line
  },
  {
    "header": {
    "po_no": "Another P.O. Number"
  },
  "line": line
  }
];

const filtered = data.filter(item => {
  return item.header.po_no === 'P.O. Number' // Evaluates to true
})