Accessing and Updating Arrays

Array elements are read and written with bracket notation and a zero-based index. Assigning to an existing index replaces the stored value.

What you will learn: the purpose of accessing and updating arrays, the rules TypeScript checks, and what its JavaScript behavior looks like in a runnable example.

An assignment modifies the array object. If two variables refer to the same array, both observe the update. Always check indexes derived from external input before using them, especially when compiler options include noUncheckedIndexedAccess.

Key distinction: TypeScript checks the declared types during development; emitted JavaScript carries out the value-level operation at runtime. A type declaration alone never validates untrusted runtime input.

Read this illustration from the first labeled part to the last. Compare it with the example and identify which parts are type checks and which parts run in JavaScript.

This self-contained example uses fixed values, so it runs without reading input or depending on an external service. Paste it into the TypeScript Playground or save it as main.ts.

TypeScript program

const marks: number[] = [60, 70, 80];
marks[1] = 75;
console.log(marks[1]);
const alias = marks;
alias[0] = 65;
console.log(marks.join(", "));

Verified output

75
65, 75, 80

Run locally: npx tsc --strict --target es2022 --module commonjs main.ts, then node main.js. For projects with a tsconfig.json, build using that configuration instead.

marks[1] changes from 70 to 75. alias refers to the same array; writing alias[0] changes marks[0] to 65. The joined result reflects both updates.

Check each step: identify the input values, follow the JavaScript operation in execution order, and match the final expression with the exact output shown. An annotation can reject invalid source before execution, but does not change the value printed.

Do not assume const makes elements immutable. const prevents reassigning marks to a different array, not mutating the array itself.

How to debug: reproduce the issue in a minimal example. For compiler errors, inspect the inferred and declared types; for a runtime surprise, log the actual values and follow their references or branches.

Use a copied array ([...marks]) and change its first value. Verify that the original array stays unchanged.

Practice routine: predict the result on paper, edit one part of the code, compile under --strict, run the JavaScript, and explain whether TypeScript caught an error or JavaScript simply behaved differently.

Accessing and Updating Arrays: Array elements are read and written with bracket notation and a zero-based index.

marks[1] changes from 70 to 75. alias refers to the same array; writing alias[0] changes marks[0] to 65. The joined result reflects both updates.

Before continuing, explain one valid use case, one common pitfall, and the separation between compile-time checking and runtime behavior for this feature.