#no-null-type

| null is forbidden in type annotation positions. Use undefined / optional (?) to represent absence.

#What it checks

| null (or null |) appearing in:

• Function parameter type annotations
• Arrow function parameter type annotations
• Variable / property type annotations with an initializer

#Why

TypeScript has two ways to represent "no value": null and undefined. Mixing them creates cognitive overhead at every call site:

// Caller now has to think: which should I pass — null or undefined?
function process(str: string | null) {}

undefined is TypeScript's canonical absence value:

• Partial<T> uses undefined, not null
• Optional chaining (?.) and nullish coalescing (??) were designed for undefined
• JSON.stringify omits undefined values but serialises null — which is the only place null is appropriate (see exception below)

Committing to undefined-only simplifies every type in the codebase.

#Examples

#✗ Incorrect

function process(str: string | null) {}

const handler = (data: { value: string | null }) => {};

let current: Item | null = null;

#✓ Correct

function process(str?: string) {}

const handler = (data: { value?: string }) => {};

let current: Item | undefined;

#Exception — JSON transformation boundary

Immediately after JSON.parse, the data may contain null. Normalise it to undefined at the boundary before passing it downstream:

// ✓  Boundary layer — null is allowed here
const raw = JSON.parse(json) as { value: string | null };
const normalized = {
  value: raw.value ?? undefined,
};

// ✓  Downstream — undefined only
process(normalized.value);

#Source

grit/no-null-type.grit