TypeScript Arrays

In TypeScript, arrays are considered to be collections of values that share a single, generic type. All elements must conform to the types specified in the array definition (e.g., string[]), or a union type if multiple element types are allowed (e.g., (string | number)[]).

Defining an array

Array types can be inferred during the initialization of a new array.

In this example, the vowels array is inferred to consist of elements of type string:

const vowels = ['a', 'e', 'i', 'o', 'u'];

vowels.push('y'); // Ok

vowels.push(7);
// Error: Argument of type 'number' is not assignable to parameter of type 'string'.

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Arrays can also be defined with the “generic” type of its elements already preset in two ways:

const vowels: string[] = ['a', 'e', 'i', 'o', 'u'];

// alternate way using Array class
const altVowels: Array<string> = ['a', 'e', 'i', 'o', 'u'];

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More than one type can be prescribed in the array definition with the “or” | operator:

const numbers: (string | number)[] = [1, '2', 3, 'four'];

// Alternative syntax
// const numbers: Array<string | number> = [1, '2', 3, 'four'];

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As long as each element in numbers is of type string or number, it is valid.

Array Types

An array type consists of the type of values inside the array followed by square brackets [].

Arrays without any initial contents may be declared as that type to tell TypeScript what will later go into them.

In this example, dates doesn’t initially include a value, so declaring it as Date[] tells TypeScript what’s allowed in it:

const dates: Date[] = [];

dates.push(new Date('2021-12-1994')); // Ok

dates.push(10241995);
// Error: Argument of type 'number' is not assignable to parameter of type 'Date'.

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