[TypeScript] Type Inference

TypeScript provides the type inference.

const b = true;
console.log(typeof b);
// boolean

const n = 3;
console.log(typeof n);
// number

const f = 3.14;
console.log(typeof f);
// number

const s = 'Hello';
console.log(typeof s);
// string

Best Common Type

When a type inference is made from several expressions, the types of those expressions are used to calculate a 'best common type'.

When no best common type is found, the resulting inference is the union array type.

let arr = [0, 1, null];
arr.push('test');  // Error: argument of type 'test' is not assignable to parameter of type 'number | null'.

Contextual Typing

Contextual typing occurs when the type of an expression is implied by its location.

Common cases include arguments to function calls, right-hand sides of assignments, type assertions, members of an object and array literals, and return statements.

The contextual type also acts as a candidate type in the best common type.

const n1 = 7;
const n2 = n1;
console.log(typeof n2);
// number