Type guards are functions that perform runtime checks and narrow TypeScript types, enabling type-safe code execution.
Primitive Type Guards
isString
Checks if a value is a string.
function isString(value: unknown): value is string
boolean - true if the value is a string
Example
const value: unknown = "hello";
if (isString(value)) {
// TypeScript knows value is a string here
console.log(value.toUpperCase()); // OK
}
isNumber
Checks if a value is a number.
function isNumber(value: unknown): value is number
boolean - true if the value is a number
Example
const value: unknown = 42;
if (isNumber(value)) {
console.log(value.toFixed(2)); // OK
}
isBoolean
Checks if a value is a boolean.
function isBoolean(value: unknown): value is boolean
boolean - true if the value is a boolean
Example
const value: unknown = true;
if (isBoolean(value)) {
console.log(value ? "yes" : "no"); // OK
}
isSymbol
Checks if a value is a symbol.
function isSymbol(value: unknown): value is symbol
boolean - true if the value is a symbol
Example
const value: unknown = Symbol("id");
if (isSymbol(value)) {
console.log(value.description); // OK
}
Collection Type Guards
isArray
Checks if a value is an array with optional type parameter for array items.
function isArray<TArray>(value: unknown): value is TArray[]
boolean - true if the value is an array
Example
const value: unknown = [1, 2, 3];
if (isArray<number>(value)) {
// TypeScript knows value is number[] here
const sum = value.reduce((a, b) => a + b, 0); // OK
}
isSet
Checks if a value is a Set.
function isSet<TSet extends Set<unknown>>(value: unknown): value is TSet
boolean - true if the value is a Set
Example
const value: unknown = new Set([1, 2, 3]);
if (isSet(value)) {
console.log(value.size); // OK
value.add(4); // OK
}
isMap
Checks if a value is a Map.
function isMap<TMap extends Map<unknown, unknown>>(value: unknown): value is TMap
boolean - true if the value is a Map
Example
const value: unknown = new Map([["key", "value"]]);
if (isMap(value)) {
console.log(value.get("key")); // OK
value.set("newKey", "newValue"); // OK
}
Object Type Guards
isObject
Checks if a value is an object (including arrays, null is excluded).
function isObject<TObject extends object>(value: unknown): value is TObject
boolean - true if the value is an object
Example
const value: unknown = { name: "Alice" };
if (isObject(value)) {
// TypeScript knows value is an object
console.log(Object.keys(value)); // OK
}
isObjectAndNotArray
Checks if a value is an object but not an array.
function isObjectAndNotArray<TObject = UnknownObject>(value: unknown): value is TObject
TObject
object
default:"UnknownObject"
The object type
boolean - true if the value is an object and not an array
Example
const obj: unknown = { name: "Alice" };
const arr: unknown = [1, 2, 3];
isObjectAndNotArray(obj); // true
isObjectAndNotArray(arr); // false
isPlainObject
Checks if a value is a plain object (created by Object literal or Object.create).
function isPlainObject<TPlainObject extends UnknownObjectWithAnyKey = UnknownObject>(
value: unknown
): value is TPlainObject
TPlainObject
object
default:"UnknownObject"
The plain object type
boolean - true if the value is a plain object
Example
class MyClass {}
isPlainObject({}); // true
isPlainObject({ name: "Alice" }); // true
isPlainObject(new MyClass()); // false
isPlainObject([]); // false
isPlainObject(null); // false
Function Type Guards
isFunction
Checks if a value is a function.
function isFunction<TFunction extends AnyFunction>(value: unknown): value is TFunction
boolean - true if the value is a function
Example
const value: unknown = () => "hello";
if (isFunction(value)) {
const result = value(); // OK
}
isAsyncFunction
Checks if a value is an async function.
function isAsyncFunction<TAsyncFunction extends AnyAsyncFunction>(
value: unknown
): value is TAsyncFunction
boolean - true if the value is an async function
Example
const value: unknown = async () => "hello";
if (isAsyncFunction(value)) {
const result = await value(); // OK
}
Browser/Web API Type Guards
Checks if a value is FormData.
function isFormData(value: unknown): value is FormData
boolean - true if the value is FormData
Example
const value: unknown = new FormData();
if (isFormData(value)) {
value.append("key", "value"); // OK
}
isFile
Checks if a value is a File.
function isFile(value: unknown): value is File
boolean - true if the value is a File
Example
const value: unknown = new File(["content"], "file.txt");
if (isFile(value)) {
console.log(value.name); // OK
console.log(value.size); // OK
}
isBlob
Checks if a value is a Blob.
function isBlob(value: unknown): value is Blob
boolean - true if the value is a Blob
Example
const value: unknown = new Blob(["data"]);
if (isBlob(value)) {
console.log(value.size); // OK
console.log(value.type); // OK
}
Utility Type Guards
isPromise
Checks if a value is a Promise.
function isPromise(value: unknown): value is Promise<unknown>
boolean - true if the value is a Promise
Example
const value: unknown = Promise.resolve("data");
if (isPromise(value)) {
const result = await value; // OK
}
isIterable
Checks if an object is iterable.
function isIterable<TIterable>(obj: object): obj is Iterable<TIterable>
The type of iterable items
boolean - true if the object is iterable
Example
const value = [1, 2, 3];
if (isIterable<number>(value)) {
for (const item of value) {
console.log(item); // OK
}
}
JSON Type Guards
isJsonString
Checks if a value is a valid JSON string.
function isJsonString(value: unknown): value is string
boolean - true if the value is a valid JSON string
Example
const value: unknown = '{"name":"Alice"}';
if (isJsonString(value)) {
const parsed = JSON.parse(value); // Safe to parse
}
isJSONSerializable
Checks if a value can be serialized to JSON.
function isJSONSerializable(value: unknown): boolean
boolean - true if the value can be serialized to JSON
Example
isJSONSerializable({ name: "Alice" }); // true
isJSONSerializable("string"); // true
isJSONSerializable(42); // true
isJSONSerializable(undefined); // false
isJSONSerializable(() => {}); // false
isSerializableObject
Checks if an object is serializable (plain object, array, or has toJSON method).
function isSerializableObject(value: unknown): boolean
boolean - true if the object is serializable
Example
isSerializableObject({ name: "Alice" }); // true
isSerializableObject([1, 2, 3]); // true
isSerializableObject(new Date()); // true (has toJSON)
isSerializableObject(new Map()); // false