pure function

example 1

let player = {
  name  : '',
  state : '',
  level : '',
  height: 0,
  age   : 0,
  point : 0
};

//impure
let impureUpdate = (name, age, height, point) => {
  let newPlayer = {
    name  : name,
    state : age > 30 ? 'senior' : 'rookie',
    level : point > 300 ? 'A' : 'B',
    height: height,
    age   : age,
    point : point
  }
  player = newPlayer;
}

impureUpdate('kobe', 20, 190, 100);
console.log('impureUpdate:', player);
/* 
console log
impureUpdate: {
  "name":"kobe",
  "state":"rookie",
  "level":"B",
  "height":190,
  "age":20,
  "point":100
}
*/

//pure
let pureUpdate = (old, latest) => {
  return {
    ...old,
    ...latest
  }
};
let updateAgeAndState = (age) => {
  let state = age > 30 ? 'senior' : 'rookie';
  return {
    age,
    state
  };
};
let updatePointAndLevel = (point) => {
  let level = point > 300 ? 'A' : 'B';
  return {
    point,
    level
  };
};
player = pureUpdate(player, {
  name  : 'jordan',
  height: 198,
  ...updateAgeAndState(32),
  ...updatePointAndLevel(350)
});
console.log('pureUpdate:', player);
/* 
console log
pureUpdate: {
  "name":"jordan",
  "state":"senior",
  "level":"A",
  "height":198,
  "age":32,
  "point":350
}
*/