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var a = [{
  name: 'xyz',
  grade: 'x'
}, {
  name: 'yaya',
  grade: 'x'
}, {
  name: 'x',
  frade: 'd'
}, {
  name: 'a',
  grade: 'b'
}];

function filterIt(arr, searchKey) {
  return arr.filter(function(obj) {
    return Object.keys(obj).some(function(key) {
      return obj[key].includes(searchKey);
    })
  });
}

console.log("find 'x'", filterIt(a,"x"));
console.log("find 'a'", filterIt(a,"a"));
console.log("find 'z'", filterIt(a,"z"));