To initialize an array in TypeScript, declare the element type and give it a value: const cities: string[] = [“Austin”, “Denver”], or const ids: string[] = [] to start empty. Array<string> means the same as string[]. For a fixed number of slots use Array.from({ length: n }, …) rather than new Array(n), which leaves holes. … Read more >>
To check if an array is null or empty in TypeScript, write if (!items || items.length === 0), or the shorter if (!items?.length), which covers null, undefined and [] in one test. The order matters: reading .length before the null check throws at runtime. This guide shows both forms running against all four cases, the … Read more >>
To remove empty strings from an array, filter them out: values.filter(Boolean) is the short version and values.filter(value => value !== “”) is the explicit one. Both return a new array and leave the original alone. If the array also contains strings that are only spaces or tabs, trim first: values.filter(value => value.trim() !== “”). The … Read more >>
To check if a TypeScript array contains a specific value, call includes(): cities.includes(“Denver”) returns true or false. That works for strings, numbers and booleans. For an array of objects it will not, because includes() compares by reference, so you test a property with some() instead: orders.some(o => o.id === “ORD-1001”). This guide covers both, plus … Read more >>
To find the maximum value in an array of objects in TypeScript, decide first what you want back. For just the number, map the property and use Math.max(): Math.max(…employees.map(e => e.salary)). For the whole object that holds it, use reduce() and keep whichever item is larger. Both are one line, and both have an edge … Read more >>
An array of objects in TypeScript is declared by describing one object and adding []: const products: Product[] = […], where Product is an interface or a type alias. Array<Product> means the same thing. Once the array is typed, the compiler checks every property you write and every property you read, which is the whole … Read more >>
To filter an array of objects in TypeScript, pass filter() a function that tests a property: orders.filter(order => order.status === “shipped”). You get a new array of the same object type, the original is untouched, and the compiler checks every property name as you type it. This guide covers filtering by one property and by … Read more >>
To sort an array in TypeScript, call sort() with a compare function: numbers.sort((a, b) => a – b) for numbers and names.sort((a, b) => a.localeCompare(b)) for text. Swap a and b for descending order. Without a compare function sort() turns every element into a string first, which is why [100, 25, 9, 1000, 3] comes … Read more >>
To filter an array in TypeScript, call filter() with a function that returns true for the elements you want to keep: temperatures.filter(temp => temp > 80). It returns a new array and never changes the original, so the result of filtering a number[] is a number[]. The one place TypeScript surprises people is removing undefined … Read more >>
