require-no-property-access-from-index-signature

Require "noPropertyAccessFromIndexSignature": true in compilerOptions to enforce bracket notation for properties defined by index signatures, distinguishing them from explicitly declared properties.

Targeted pattern scope

The compilerOptions.noPropertyAccessFromIndexSignature field in any tsconfig*.json file.

What this rule reports

This rule reports when compilerOptions.noPropertyAccessFromIndexSignature is absent or explicitly set to false.

Why this rule exists

TypeScript allows two ways to access a property on an object:

Dot notation: obj.key — suggests key is a known, declared property
Bracket notation: obj["key"] — suggests the key may not be declared

When an object type has an index signature, TypeScript normally allows both notations interchangeably:

interface Config {
    strict: boolean; // declared property
    [key: string]: unknown; // index signature
}

declare const cfg: Config;
cfg.strict; // declared — fine
cfg.unknownKey; // index signature — TypeScript allows this without the flag

With noPropertyAccessFromIndexSignature: true, TypeScript requires bracket notation for any access that is satisfied by an index signature rather than a declared property:

cfg.strict; // ✅ declared property — dot notation allowed
cfg["unknownKey"]; // ✅ index-signature access — bracket notation required
cfg.unknownKey; // ❌ error — must use bracket notation for index-sig access

This distinction has two practical benefits:

Visual clarity: Dot notation signals "I know this property exists". Bracket notation signals "this may or may not exist at runtime".
Interaction with noUncheckedIndexedAccess: When both flags are enabled, bracket-notation index accesses produce T | undefined rather than T, making the potential absence of the value explicit.

The auto-fixer adds "noPropertyAccessFromIndexSignature": true to compilerOptions.

❌ Incorrect

{
    "compilerOptions": {
        "strict": true,
        "noUncheckedIndexedAccess": true
    }
}

noPropertyAccessFromIndexSignature is absent; dot notation is silently allowed for index-signature accesses.

{
    "compilerOptions": {
        "noPropertyAccessFromIndexSignature": false
    }
}

Explicitly disabled.

✅ Correct

{
    "compilerOptions": {
        "strict": true,
        "noUncheckedIndexedAccess": true,
        "noPropertyAccessFromIndexSignature": true
    }
}

Index-signature accesses must use bracket notation.

When not to use it

Disable this rule in projects that access index-signature properties with dot notation throughout the codebase and where migrating to bracket notation is not immediately feasible. The rule pairs best with noUncheckedIndexedAccess: true for maximum benefit.

Package documentation

Rule catalog ID: R035

Targeted pattern scope​

What this rule reports​

Why this rule exists​

❌ Incorrect​

✅ Correct​

When not to use it​

Package documentation​

Further reading​