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
}]