Mastering TypeScript/

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Definite Assignment

Definite Assignment

Learn to handle variable use and definition, including errors and definite assignment assertion syntax.

We'll cover the following...

Introduction to definite assignment

Variables in JavaScript are defined by using the var keyword. Unfortunately, the JavaScript runtime is very lenient on where these definitions occur and will allow a variable to be used before it has been defined.

Consider the following JavaScript code:

// This line logs the value of the variable "aValue" to the console, before it has been defined
console.log("aValue = " + aValue);
// This line declares the variable "aValue" and assigns it the value of 1
var aValue = 1;
// This line logs the value of the variable "aValue" to the console, after it has been defined and assigned a value
console.log("aValue = " + aValue);
Logging undefined variable value in JavaScript

We start the code by logging the value of a variable named aValue to the console. Note, however, that we only declare the aValue variable on line 5 of the above code snippet.

As we can see from this output, the value of the aValue variable before it had been declared is undefined. This can obviously lead to unwanted behavior, and any good JavaScript programmer will check that a variable is not undefined before attempting to use ...