Mastering TypeScript/

...

/

The let and const Keywords

The let and const Keywords

Learn how to define a variable's scope and constants using the let and const keywords in TypeScript.

We'll cover the following...

The let keyword

The fluid nature of JavaScript variables can sometimes cause errors when we inadvertently define variables with the same name but in a different scope within a code block.

Consider the following TypeScript code:

TypeScript 4.9.5
// Declare a variable called index with a type of number and assign it the value 0
var index: number = 0;
// If index is equal to 0, create a new block scope with a new variable also called index, but with a type of number and value of 2, and log its value
if (index == 0) {
  var index: number = 2;
  console.log(`index = ${index}`);
}
// Log the value of index
console.log(`index = ${index}`);
Variable index redeclared in the if-block

  • On line 2, we define a variable named index of type number using the var keyword and assign it a value of 0.

  • We then test if this value is equal to 0 on line 5, and if it is, we enter a code block.

  • The first statement in this code block on line 6 defines a variable named index of type number and ...