Initialize an empty Boolean Array in TypeScript

Initialize an empty Boolean Array in TypeScript #

You can initialize an empty boolean array as const arr: boolean[] = []. The array can contain zero or more elements of type boolean. Trying to add an element of a different type to the array causes the type checker to throw an error.

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// ✅ First approach (better)
const arr: boolean[] = [];

// ✅ Second approach
const arr2 = [] as boolean[];

We initialized an empty boolean array as boolean[]. The array can contain zero or more booleans, but if we try an element of a different type, we'd get an error.

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const arr: boolean[] = [];

arr.push(true); // ✅ OK

// ⛔️ Error: Argument of type 'string' is not
// assignable to parameter of type 'boolean'.ts(2345)
arr.push('hello');

You can use this approach to initialize an empty array of any type.

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const strArray: string[] = [];
strArray.push('hello');

const objArray: { id: number }[] = [];
objArray.push({ id: 1 });

const twoDimensionalArray: string[][] = [];
twoDimensionalArray.push(['a', 'b', 'c']);

Note that the syntax is boolean[] and not [boolean]. This is often a source of confusion, but boolean[] is an array of zero or more elements of type boolean, and [boolean] is a tuple with a single boolean element.

An alternative way to initialize an empty boolean array is to use a type assertion.

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const arr2 = [] as boolean[];

arr2.push(false); // ✅ OK

// ⛔️ Error: Argument of type 'string' is
// not assignable to parameter of type 'boolean'.ts(2345)
arr2.push('hello');

Type assertions are used when we have information about the type of a value that TypeScript can't know about.

Having the array typed as any[] effectively turns off any type checking, so it's not what we want most of the time.