Immutability

immutability.js

var obj;
var anotherObj;

// A mutable example.
obj = {
  'field': 'value'
};

// An example of unintended mutability. By default, JavaScript assigns a *reference* to anotherObj, pointing
// at the *same* object. Therefore, when we go to modify anotherObj, we accidentally modify obj too.
anotherObj = obj;
anotherObj.field = 'another value';

// Prints 'another value'.
console.log(obj.field);

// Prints 'another value'.
console.log(anotherObj.field);

// Prints true
console.log(obj === anotherObj);

// Can we do this immutably?
obj = {
  field: 'value' 
};

anotherObj = {...obj};
anotherObj.field = 'another value';

// Prints 'value'.
console.log(obj.field);

// Prints 'another value'.
console.log(anotherObj.field);

// Prints false.
console.log(obj === anotherObj);