wiki/content/20201126104931-nouncheckedindexaccess.md

date	id	title
2020-11-26	81de1606-f1e2-41e6-a386-103e110436cd	noUncheckedIndexAccess

Default

false

Description

TypeScript supports 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 array 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 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 PR¹.

Footnotes

---

1. https://github.com/microsoft/TypeScript/pull/39560 ↩︎