String Methods

String methods such as trim, slice, includes, startsWith, and replace help inspect or transform text.

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

Strings are immutable and method results should be stored or used. Index positions count UTF-16 code units. Choose the operation with the right contract: includes searches for a substring, while startsWith checks a prefix.

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 raw = "  TypeScript basics  ";
const text = raw.trim();
console.log(text.startsWith("Type"));
console.log(text.slice(0, 10));
console.log(text.replace("basics", "guide"));

Verified output

true
TypeScript
TypeScript guide

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.

trim removes surrounding spaces. startsWith returns true. slice(0,10) selects TypeScript. replace creates TypeScript guide.

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 slice end indexes with character counts in every case; the end position is exclusive. replace without a global regular expression handles only the first matching occurrence.

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.

Take " Learn TS ", trim it, test includes("TS"), and replace TS with TypeScript.

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.

String Methods: String methods such as trim, slice, includes, startsWith, and replace help inspect or transform text.

trim removes surrounding spaces. startsWith returns true. slice(0,10) selects TypeScript. replace creates TypeScript guide.

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