08 - Const and Let

Section 08: const and let

• The Keyword const and let
• [Exercise] A Constant Exercise of Letting Variables Be Variables
• [Exercise] Const/Let Refactoring

The Keyword const and let

const name = 'Jane';
let title = 'Software Engineer';
let hourlyWage = 40;

// some time later
title = 'Senior Software Engineer';
hourlyWage = 45;

We should always use const and let to declare variables instead of using var. Use the const and let make our codes more legible.

• The keyword let declare variables and the value of the variable will change over time.
• The keyword const declare variables and the value of the variable will never change.

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[Exercise] A Constant Exercise of Letting Variables Be Variables

Question

Imagine that you are building an application to manage a user's Facebook profile. A profile might have a name, age, and dateOfBirth.

Declare three variables with these same names, making sure to use const or let depending on whether you expect the value to change over time.

Solution

const name = 'Peter';
const dateOfBirth = '900422';
let age = 25;

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[Exercise] Const/Let Refactoring

Question

The following code uses var instead of const and let. Refactor the function to use the new keywords. Be sure to consider whether the variable should be declared using const or let depending on whether the variable gets reassigned or not.

var statuses = [
  { code: 'OK', response: 'Request successful' },
  { code: 'FAILED', response: 'There was an error with your request' },
  { code: 'PENDING', response: 'Your reqeust is still pending' }
];
var message = '';
var currentCode = 'OK';

for (var i = 0; i < statuses.length; i++) {
  if (statuses[i].code === currentCode) {
    message = statuses[i].response;
  }
}

Solution

const statuses = [
  { code: 'OK', response: 'Request successful' },
  { code: 'FAILED', response: 'There was an error with your request' },
  { code: 'PENDING', response: 'Your reqeust is still pending' }
];
let message = '';
let currentCode = 'OK';

for (let i = 0; i < statuses.length; i++) {
  if (statuses[i].code === currentCode) {
    message = statuses[i].response;
  }
}

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