freeCodeCamp/curriculum/challenges/english/02-javascript-algorithms-and-data-structures/es6/prevent-object-mutation.english.md

id	title	challengeType	forumTopicId
598f48a36c8c40764b4e52b3	Prevent Object Mutation	1	301207

Description

As seen in the previous challenge, const declaration alone doesn't really protect your data from mutation. To ensure your data doesn't change, JavaScript provides a function Object.freeze to prevent data mutation. Once the object is frozen, you can no longer add, update, or delete properties from it. Any attempt at changing the object will be rejected without an error.

let obj = {
  name:"FreeCodeCamp",
  review:"Awesome"
};
Object.freeze(obj);
obj.review = "bad"; // will be ignored. Mutation not allowed
obj.newProp = "Test"; // will be ignored. Mutation not allowed
console.log(obj); 
// { name: "FreeCodeCamp", review:"Awesome"}

Instructions

In this challenge you are going to use Object.freeze to prevent mathematical constants from changing. You need to freeze the MATH_CONSTANTS object so that no one is able to alter the value of PI, add, or delete properties.

Tests

tests:
  - text: Do not replace <code>const</code> keyword.
    testString: getUserInput => assert(getUserInput('index').match(/const/g));
  - text: <code>MATH_CONSTANTS</code> should be a constant variable (by using <code>const</code>).
    testString: getUserInput => assert(getUserInput('index').match(/const\s+MATH_CONSTANTS/g));
  - text: Do not change original <code>MATH_CONSTANTS</code>.
    testString: getUserInput => assert(getUserInput('index').match(/const\s+MATH_CONSTANTS\s+=\s+{\s+PI:\s+3.14\s+};/g));
  - text: <code>PI</code> equals <code>3.14</code>.
    testString: assert(PI === 3.14);

Challenge Seed

function freezeObj() {
  'use strict';
  const MATH_CONSTANTS = {
    PI: 3.14
  };
  // change code below this line


  // change code above this line
  try {
    MATH_CONSTANTS.PI = 99;
  } catch(ex) {
    console.log(ex);
  }
  return MATH_CONSTANTS.PI;
}
const PI = freezeObj();

Solution

function freezeObj() {
  'use strict';
  const MATH_CONSTANTS = {
    PI: 3.14
  };
  // change code below this line
  Object.freeze(MATH_CONSTANTS);

  // change code above this line
  try {
    MATH_CONSTANTS.PI = 99;
  } catch(ex) {
    console.log(ex);
  }
  return MATH_CONSTANTS.PI;
}
const PI = freezeObj();