Learning TypeScript

Chapter 6. Arrays

Arrays and tuples

One flexible and one fixed

Choose your adventure

JavaScript arrays are wildly flexible and can hold any mixture of values inside:

const elements = [true, null, undefined, 42];

elements.push("even", ["more"]);
// Value of elements: [true, null, undefined, 42, "even", ["more"]]

In most cases, though, individual JavaScript arrays are intended to hold only one specific type of value. Adding values of a different type may be confusing to readers, or worse, the result of an error that could cause problems in the program.

TypeScript respects the best practice of keeping to one data type per array by remembering what type of data is initially inside an array, and only allowing the array to operate on that kind of data.

In this example, TypeScript knows the warriors array initially contains string typed values, so while adding more string typed values is allowed, adding any other type of data is not:

const warriors = ["Artemisia", "Boudica"];

// Ok: "Zenobia" is a string
warriors.push("Zenobia");

warriors.push(true);
//            ~~~~
// Argument of type 'boolean' is not assignable to parameter of type 'string'.

You can think of TypeScript’s inference of an array’s type from its initial members as similar to how it understands variable types from their initial values. TypeScript generally tries to understand the intended types of your code from how values are assigned, and arrays are no exception.

Array Types

As with other variable declarations, variables meant ...