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Non-nullable types

TypeScript 2.0 introduced what are known as non-nullable types. TypeScript used to consider null and undefined to be valid values of every type.

The following diagram represents the values that can be assigned to the type number when non-nullable types are disabled:

As we can see in the preceding diagram, undefined and null are allowed as values of the type number, together with the NaN value and all the possible numbers.

NaN, standing for not a number, is a numeric data type value representing an undefined or non-representable value, especially in floating-point calculations. Systematic use of NaNs was introduced by the IEEE 754 floating-point standard in 1985, along with the representation of other non-finite quantities such as infinities.

The following code snippet demonstrates how undefined and null are allowed as values of the type number when non-nullable types are disabled:

let name: string;
name = "Remo"; // OK
name = null; // OK
name = undefined; // OK

The same can be said about all other types:

let age: number;
age = 28; // OK
age = null; // OK
age = undefined; // OK

let person: { name: string, age: number};
person = { name: "Remo", age: 28 }; // OK
person = { name: null, age: null }; // OK
person = { name: undefined, age: undefined }; // OK
person = null;       // OK
person = undefined;  // OK

When non-nullable types are enabled, the values null and undefined are considered independent types and stop being considered as valid values of the type number:

The following code snippet demonstrates how undefined and null are not allowed as values of the type number when non-nullable types are enabled:

let name: string;
name = "Remo"; // OK
name = null; // Error
name = undefined; // Error

The same can be said about all other types:

let age: number;
age = 28; // OK
age = null; // Error
age = undefined; // Error

let person: { name: string, age: number};
person = { name: "Remo", age: 28 }; // OK
person = { name: null, age: null }; // Error
person = { name: undefined, age: undefined }; // Error
person = null; // Error
person = undefined; // Error

We can enable non-nullable types by using the --strictNullChecks compilation flag:

tsc -strictNullChecks file.ts

When non-nullable types are enabled, we can use union types to create nullable versions of a type:

type NullableNumber = number | null;