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TypeScript Array Type

Summary: in this tutorial, you’ll learn about the TypeScript array type and its basic operations.

Introduction to TypeScript array type

A TypeScript array is an ordered list of data. To declare an array that holds values of a specific type, you use the following syntax:

let arrayName: type[];

For example, the following declares an array of strings:

let skills: string[] = [];

And you can add one or more strings to the array:

skills[0] = "Problem Solving";
skills[1] = "Programming";

or use the push() method:

skills.push('Software Design');

The following declares a variable and assigns an array of strings to it:

let skills = ['Problem Sovling','Software Design','Programming'];

In this example, TypeScript infers the skills array as an array of strings. It is equivalent to the following:

let skills: string[];
skills = ['Problem Sovling','Software Design','Programming'];

After you define an array of a specific type, TypeScript will prevent you from adding incompatible values. For example, the following will cause an error:

skills.push(100);

… because we’re trying to add a number to the string array.

Error:

Argument of type 'number' is not assignable to parameter of type 'string'.

When you extract an element from the array, TypeScript infers the type of the array element. For example:

let skill = skills[0];
console.log(typeof(skill));

Output:

string

In this example, we extract the first element of the skills array and assign it to the skill variable.

Since an element in a string array is a string, TypeScript infers the type of the skill variable to string as shown in the output.

TypeScript array properties and methods

TypeScript arrays have the same properties and methods as JavaScript arrays. For example, the following uses the length property to get the number of elements in an array:

let series = [1, 2, 3];
console.log(series.length); // 3

You can use all the useful array methods such as forEach(), map(), reduce(), and filter(). For example:

let series = [1, 2, 3];
let doubleIt = series.map(e => e* 2);
console.log(doubleIt);

Output:

[ 2, 4, 6 ]

Storing values of mixed types

The following illustrates how to define an array that holds both strings and numbers:

let scores = ['Programming', 5, 'Software Design', 4];

In this case, TypeScript infers the scores array as an array of string | number. It’s equivalent to the following:

let scores : (string | number)[];
scores = ['Programming', 5, 'Software Design', 4];

Summary

  • In TypeScript, an array is an ordered list of values.

  • Use the let arr: type[] syntax to declare an array of a specific type. Adding a value of a different type to the array will result in an error.

  • An array can store values of mixed types. Use the arr: (type1 | type2) [] syntax to declare an array of values with mixed types (type1, and type2)