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TypeScript checks how types fit together before your JavaScript runs; it does not change JavaScript’s runtime behavior. If you already know JavaScript, a useful refresher is to revisit everyday types, function signatures and object shapes, then work through unions and narrowing before moving on to generics.
What TypeScript does—and what it does not do
TypeScript is a static typechecker for JavaScript. It checks a program’s type relationships before runtime, helping catch mismatches such as passing a number to a function that expects a string. The code still runs according to JavaScript’s runtime rules: TypeScript does not make runtime values safe merely because they have a declared type.
The TypeScript Handbook is designed as a guide for everyday programmers, not as a complete language specification. It gives a strong overview of common syntax and behavior; consult the more detailed reference pages when you need precise rules or edge cases. If you are not yet comfortable with JavaScript, the Handbook recommends learning JavaScript first.
Review everyday types and inference
TypeScript uses lowercase names for JavaScript’s familiar primitive types: string, number, and boolean. It also has the primitive type symbol. You can annotate a value when the type is not obvious or when the annotation clarifies intent:
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let username: string = "Ari";
let count: number = 3;
let isReady: boolean = true;For a local variable whose initial value makes the type clear, you can usually let TypeScript infer it instead of repeating the type mechanically:
let retries = 3; // inferred as numberAnnotations are often most useful at boundaries—such as function parameters and return types—where they make expectations clear and let TypeScript check how code is used. The Everyday Types chapter covers common types and where annotations belong.
Make function expectations explicit
Annotating a parameter lets TypeScript check calls to the function. A return annotation describes what the function returns:
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function greet(name: string): string {
return `Hello, ${name}`;
}
greet("Mina");
// greet(42); // Error: number is not assignable to stringUse void when a function does not return a meaningful value for its caller. It describes the function’s type-level return behavior; it does not alter the function’s JavaScript runtime behavior.
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function logMessage(message: string): void {
console.log(message);
}Describe object shapes
An object type describes the properties a value is expected to have and the types of those properties. For a one-off shape, write it inline:
function printUser(user: { name: string; active: boolean }): void {
console.log(user.name, user.active);
}If the same shape needs a name, either an interface or a type alias can express it:
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interface User {
name: string;
active: boolean;
}
type UserRecord = {
name: string;
active: boolean;
};Choose the form that makes the surrounding code easiest to understand. The key idea is the shape: which properties are expected and what types they have. The Handbook’s Objects chapter explores object types in more detail.
Use unions with control-flow narrowing
A union such as string | number means a value may be either type. Code cannot safely use an operation that applies to only one alternative until it checks which type is present:
function format(value: string | number): string {
if (typeof value === "string") {
return value.toUpperCase();
}
return value.toFixed(2);
}TypeScript follows recognized JavaScript checks through control flow and narrows the type in the corresponding branch. Besides checks such as typeof, the Handbook demonstrates narrowing with in and instanceof. For example, "swim" in animal can distinguish object alternatives based on whether they have a property, while pet instanceof Fish can narrow based on a constructor check.
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Narrowing is not the same as validating arbitrary external data. If a value comes from an API, file, or other untrusted source, a type annotation alone does not prove that the runtime value has the shape you expect; your program still needs appropriate runtime checks.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Reach for generics when types should stay connected
A generic type parameter lets a function express a relationship between its input and output instead of discarding useful type information. For instance, a first-element helper can preserve the array’s element type and account for an empty array:
function firstElement<T>(items: T[]): T | undefined {
return items[0];
}
const firstName = firstElement(["Mina", "Ari"]); // string | undefined
const firstCount = firstElement([4, 7]); // number | undefinedHere, T represents the element type inferred from the array argument. The return type stays tied to that element type, with undefined included because an array may be empty. This input/output correspondence is a central use of generics; they are most helpful when a reusable function should preserve such a relationship, rather than when a simpler concrete type says enough.
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A practical order for refreshing TypeScript
The Handbook covers many topics, but this sequence is a useful route through the basics rather than a required syllabus:
- Primitive types and inference: Review
string,number,boolean, and when to rely on inferred local types. - Function signatures: Practice parameter and return annotations, including
void. - Object shapes: Describe properties inline, then compare named interfaces and type aliases.
- Unions and narrowing: Read a union safely by checking which alternative is present.
- Generics: Preserve useful input/output relationships in reusable functions.
The official Handbook is the default starting point for this route. Its concise walkthrough is paired with reference material for details the overview does not fully explore.
Avoid three common type traps
- Boxed types: Prefer primitive lowercase types—
string,number,boolean, andsymbol—overString,Number,Boolean,Symbol, or globalObjectas ordinary annotation types. The official Do’s and Don’ts says the boxed forms are almost never appropriate. anyas a shortcut:anyeffectively turns off type checking for that value, so mistakes can pass unchecked. When a value could be anything and must be examined before use, preferunknown; TypeScript requires you to narrow it before performing type-specific operations.- Assuming a type annotation validates data: Types guide static checking, but they do not establish that untrusted runtime data is valid. Check such data at runtime before relying on its shape or contents.
Optional structured practice
If you prefer a physical, exercise-oriented companion, No Starch Press lists Total TypeScript: The Essentials by Matt Pocock with Taylor Bell as a 432-page print book published in April 2026. The publisher describes a progression through primitive types, unions, objects, classes, basic generics, and exercises. It is an optional way to add structured practice, not a prerequisite or replacement for the free Handbook.
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