Quick Overview
`strict` is a bundle, not a single flag: it turns on `noImplicitAny`, `strictNullChecks` and six others at once. `noUncheckedIndexedAccess` sits outside it, making `arr[0]` admit it may be `undefined`.
TypeScript's type checking rigor is configured inside tsconfig.json. Enabling strict compiler flags turns TypeScript into an uncompromising production quality gate.
"strict": trueEnabling "strict": true automatically turns on a family of strict type checking flags:
noImplicitAny: Throws a compile error if a variable or parameter type cannot be inferred and defaults implicitly to any.strictNullChecks: Ensures null and undefined are not assignable to other types unless explicitly unioned.strictFunctionTypes: Enforces strict bivariance checks on function parameters.strictBindCallApply: Ensures arguments passed to .call(), .apply(), and .bind() match underlying function signatures.noUncheckedIndexedAccessBy default, reading dynamic object or array indices (e.g. arr[0] or dict["key"]) returns the value type. Enabling noUncheckedIndexedAccess: true forces array/object lookups to include | undefined:
// tsconfig.json -> "noUncheckedIndexedAccess": true
const list: string[] = ["Alex"];
const item = list[5]; // Inferred type: string | undefined (Forces safe checking!)
{
"compilerOptions": {
"target": "ES2022",
"module": "NodeNext",
"moduleResolution": "NodeNext",
"strict": true,
"noImplicitAny": true,
"strictNullChecks": true,
"noUncheckedIndexedAccess": true,
"noImplicitReturns": true,
"skipLibCheck": true
}
}
strict: true enabled on all codebases!Enable strict flags on every project. Next, let's explore ECMAScript Decorators in TypeScript 5.0+!
TypeScript erases entirely at build time, so a `string` annotation produces no runtime check whatsoever. A value arriving from an API is whatever the server actually sent, regardless of what the annotation claims.
`let x = "GET"` infers `string` because the binding can change; `const x = "GET"` infers the literal `"GET"`. That widening rule is why object properties need `as const` to stay narrow.
`string[]` is any number of strings; `[string, number]` is exactly two, in that order. `readonly` removes the mutating methods at compile time, so `push` becomes a type error rather than a runtime surprise.
