TypeScript Basics: Arrays

• Array types are marked with []. An array of numbers is number[].

• >

  let numbers: number[] = [1, 2, 3];
  numbers[1];

  Result:

  2

• Arrays can contain any type: boolean[] is an array of booleans, etc.

• >

  let bools: boolean[] = [true, false];
  bools[1];

  Result:

  false

• The type boolean[] has two parts. Overall, it's an array. But inside the array, there are booleans.

• For now, we'll only consider arrays that hold exactly one type, like boolean[] or string[]. Later, we'll see arrays with mixed types, like an array of strings and numbers.

• The next example contains a type error: our variable is a string[], but we try to put a 3 inside the array.

• >

  let strings: string[] = ['a', 'b', 3];
  strings;

  Result:

  type error: Type 'number' is not assignable to type 'string'.

• Remember that types are always checked. Assigning to a new variable preserves the type, even if it's inferred.

• >

  let strings: string[] = ['a', 'b', 'c'];
  let values = strings;
  values;

  Result:

  ['a', 'b', 'c']

• >

  let strings: string[] = ['a', 'b', 'c'];
  let values = strings;
  let numbers: number[] = values;
  numbers;

  Result:

  type error: Type 'string[]' is not assignable to type 'number[]'.
    Type 'string' is not assignable to type 'number'.