JavaScript

2Block-Scoped Declarations: let and const

WHAT'S IN THIS CHAPTER?

• An introduction to let and const
• Definition of “block scope” with examples
• Shadowing and hoisting: the Temporal Dead Zone
• Using const for variables that shouldn't change
• Creating global variables that aren't on the global object
• Using block scope in loops

CODE DOWNLOADS FOR THIS CHAPTER

You can download the code for this chapter at https://thenewtoys.dev/bookcode or https://www.wiley.com/go/javascript-newtoys.

In this chapter you'll learn how the new let and const declarations work and what problems they solve. Throughout the chapter, you'll see some behaviors of var that have proved problematic, and you'll learn how let and const solve those problems. You'll see how let and const provide true block scope and prevent confusion caused by repeated declarations or by using a variable before you initialize it. You'll discover how block scope means you can use let to avoid the traditional “closures in loop” problem, and how const lets you create constants: “variables” whose values cannot change. You'll learn how let and const avoid creating even more properties on the already-overburdened global object. In short, you'll learn why let and const are the new var, and why var no longer has a place in modern JavaScript programming.

AN INTRODUCTION TO LET AND CONST

Like var, let declares variables:

let x = 2;
x += 40;
console.log(x); // 42

You can use let anywhere you can use var. Just as with var, you ...