Declare Array that only accepts specific Values in TS

Declare Array that only accepts specific Values in TS #

Use a union type to declare an array that only accepts specific values, e.g. const arr2: ('a' | 'b' | 'c')[] = []. A union type is formed from two or more other types or literals. The array in the example can only contain the strings a, b and c.

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type NumbersLessThan5 = 1 | 2 | 3 | 4;

const arr: NumbersLessThan5[] = [];

arr.push(1);
arr.push(2);
arr.push(3);
arr.push(4);

We used a union type to create an array that only accepts specific values.

The array in the example can only contain the numbers 1-4. If we try to add a number that is not contained in the union type to the array, we'd get an error.

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type NumbersLessThan5 = 1 | 2 | 3 | 4;

const arr: NumbersLessThan5[] = [];

// ⛔️ Error: Argument of type '5' is not
// assignable to parameter of type 'NumbersLessThan5'.ts(2345)
arr.push(5);

This works in the same way when using string literals.

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type FirstThreeLetters = 'a' | 'b' | 'c';

const arr2: FirstThreeLetters[] = [];

arr2.push('a');
arr2.push('b');
arr2.push('c');

// ⛔️ Error: Argument of type '"d"' is not
// assignable to parameter of type 'FirstThreeLetters'.ts(2345)
arr2.push('d');

The values do not have to be of the same type. You can use the same approach in the unlikely scenario that you have mixed arrays.

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type Mixed = 'a' | 'b' | 1;

const arr2: Mixed[] = [];

arr2.push('a');
arr2.push('b');

arr2.push(1);

Union types are quite useful in TypeScript, in fact the boolean type is just an alias for the union true | false.