[Dilettante_Pro]

Он дает пустой массив.

???

<input type="text" id = "textarea" value = "Игорь">
<script>
let textarea = document.getElementById("textarea");
let list = [{name:"Вася",age:"23"},
{name:"Игорь", age:"25"}];
function filter(text, parameter){

let a = [];

for(let v of list){
if(v[parameter].indexOf(text) != -1)
a.push(v);
}

return a;
}

 let b = filter(textarea.value, "name");
alert(JSON.stringify(b));
</script>

<input type="text" id = "textarea" value = "Игорь">
<script>
let textarea = document.getElementById("textarea");
let list = [{name:"Вася",age:"23"},
{name:"Игорь", age:"25"}];
function filter(list, text, parameter) {
  return list.filter(el=>el[parameter].includes(text));
}

let b = filter(list ,textarea.value, "name");
alert(JSON.stringify(b));
</script>