let arr = [{id:1, name:"John"}, {id:2, name: "Doe"}];

let item = arr.find((x)=> x.id === 2);

​

//the array is not changed!

item = Object.assign({}, {id:3, name: "Do"});

console.log(arr);

​

let item2 = arr.find((x)=> x.id === 2);

//the array is changed!

Object.assign(item2,  {id:3, name: "Do"});

console.log(arr);