need to Convert a string to objects [Angular 6]

Question

I am trying to convert a string in an object which is having function a value. like this

for (let i = 0; i < this.dataSource.data.length; i++) {
  let control = <FormArray>this.formShipment.controls['User'];
  var a;
  if (this.dataSource.data[i].active == '1') {
    a = true
  } else {
    a = false
  }
  var s= { 'custom_value'+i : new FormControl(a) }
  let objectValue = this._formBuilder.group(s);
  var b = { 'value': objectValue }
  control.push(objectValue);
}

So what i have tried is

for (let i = 0; i < this.dataSource.data.length; i++) {
  let control = <FormArray>this.formShipment.controls['User'];
  var a;
  if (this.dataSource.data[i].active == '1') {
    a = true
  } else {
    a = false
  }
  var k='{ "custom_value'+i+'"  : new FormControl(a) }'
  var m= JSON.parse(k)
  let objectValue = this._formBuilder.group(m);
  var b = { 'value': objectValue }
  control.push(objectValue);
  console.log(objectValue)
}

So please help that how to handle the error in it.

Thanks in Advance!