Скачать или смотреть Strings in Typescript

Here’s a full guide to strings in TypeScript, with examples for different situations and usage patterns

🔹 1. Basic String Declaration

In TypeScript, you can define strings in three main ways:

let firstName: string = "John"; // double quotes
let lastName = 'Bigley'; // single quotes (inferred as string)
let greeting = `Hello, ${firstName}`; // template literal


Explanation:

You can use single ('), double (") or backticks (`).

Backticks are used for template literals — they allow multi-line strings and embedded expressions.

🔹 2. Template Literals

Template literals are powerful for dynamic strings.

let age = 30;
let info = `Name: ${firstName} ${lastName}, Age: ${age}`;
console.log(info);


✅ Output:

Name:John Bigley, Age: 30


Use cases:

Embedding variables

Multi-line text

Generating formatted output

🔹 3. String Methods

TypeScript provides all JavaScript string methods, but with type safety.

let city = "Lviv";

console.log(city.toUpperCase()); // LVIV
console.log(city.toLowerCase()); // lviv
console.log(city.includes("v")); // true
console.log(city.charAt(2)); // i


TypeScript benefit: It infers that city is a string, so methods like .toUpperCase() are available automatically.

🔹 4. String Concatenation

You can combine strings using + or template literals:

let fullName = firstName + " " + lastName;
let sentence = `${fullName} is learning TypeScript.`;

🔹 5. String Union Types

You can limit a variable to a specific set of string values.

type Direction = "up" | "down" | "left" | "right";

let move: Direction = "up"; // ✅ OK
move = "left"; // ✅ OK
move = "forward"; // ❌ Error


Use case:
Useful in APIs, UI states, or enums for predictable string values.

🔹 6. String Literal Types in Functions
function setStatus(status: "success" | "error" | "loading") {
console.log(`Status: ${status}`);
}

setStatus("success"); // ✅ OK
setStatus("failed"); // ❌ Error


This pattern enforces only valid string options.

🔹 7. String Arrays
let fruits: string[] = ["apple", "banana", "orange"];
fruits.push("mango"); // ✅
fruits.push(42); // ❌ Error


Explanation:
TypeScript ensures all array elements are strings.

🔹 8. Strings with Optional or Null Values
let middleName: string | null = null;

if (middleName) {
console.log(middleName.toUpperCase());
}


Note:
To safely handle nullable strings, you can use optional chaining or condition checks.

🔹 9. Function Parameters and Returns
function greet(name: string): string {
return `Hello, ${name}!`;
}

console.log(greet("Orest")); // ✅ Hello, Orest!

🔹 10. Type Inference for Strings
let course = "TypeScript"; // inferred as string
course = 123; // ❌ Error


TypeScript infers the variable type from the first assignment.