Multidimensional Arrays

A two-dimensional array is an array of arrays. The type number[][] means each row is a number[]; rows may have different lengths.

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

Access one dimension at a time: grid[row][column]. Nested loops are natural for processing every cell. JavaScript does not require a rectangular shape, so check row lengths before accessing columns derived from external input.

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 grid: number[][] = [[1, 2], [3, 4], [5, 6]];
console.log(grid[1][0]);
const rowTotals = grid.map(row => row.reduce((a, b) => a + b, 0));
console.log(rowTotals.join(", "));

Verified output

3
3, 7, 11

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.

grid[1] picks the second row [3,4], then [0] selects 3. Mapping across rows produces totals 3, 7, and 11.

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 confuse grid[1][0] with grid[0][1], and do not assume all rows have identical lengths unless your program enforces that rule.

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.

Create a 3×3 matrix and calculate the sum of the first column by iterating through rows.

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.

Multidimensional Arrays: A two-dimensional array is an array of arrays.

grid[1] picks the second row [3,4], then [0] selects 3. Mapping across rows produces totals 3, 7, and 11.

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