noUncheckedIndexAccess
Default
~false~
Description
TypeScript supports [TypeScript Index Signatures](TypeScript Index Signatures). These signatures are a way to signal to the type system that users can access arbitrarily-named properties:
interface Options {
path: string;
permissions: number;
// Extra properties are caught by this index signature.
[propName: string]: string | number;
}
function checkOptions(opts: Options) {
opts.path // string
opts.permissions // number
// These are all allowed too!
// They have the type 'string | number'.
opts.yadda.toString();
opts["foo bar baz"].toString();
opts[Math.random()].toString();
}
When the —noUncheckedIndexAccess flag is used, every property access (like foo.bar) or indexed access (like foo[“bar”]) that ends up resolving to an index signature is considered potentially undefined. Ie: opts.yadda will have the type string | number | undefined as opposed to just string | number. If you need to access that property, you’ll either have to check for its existence first or use a non-null assertion operator (the postfix ! character):
// Checking if it’s really there first.
if (opts.yadda) {
console.log(opts.yadda.toString());
}
// Basically saying “trust me I know what I’m doing”
// with the ’!’ non-null assertion operator.
opts.yadda!.toString();
One consequence of using —noUncheckedIndexedAccess is that indexing into an [TypeScript Arrays](TypeScript Arrays) is also more strictly checked, even in a bounds-checked loop:
function screamLines(strs: string[]) {
// this will have issues
for (let i = 0; i < strs.length; i++) {
console.log(strs[i].toUpperCase());
// ~~~~~~~
// error! Object is possibly ‘undefined’.
}
}
If you don’t need the indexes, you can iterate over individual elements by using a [JavaScript For Of](JavaScript For Of) loop or a forEach call:
function screamLines(strs: string[]) {
// this works fine
for (const str of strs) {
console.log(str.toUpperCase());
}
// this works fine
strs.forEach(str => {
console.log(str.toUpperCase());
});
}
This flag can be handy for catching out-of-bounds errors, but it might be noisy for a lot of code, so it is not automatically enabled by the —strict flag. More information can be found in the PR[fn:pr].