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Initialize an Array of Objects using for loop # log(arr2); We used the Array() constructor to create an array of 2 empty elements. The for loop allows us to iterate over the array and assign each empty element to an object.
Initialize Typed variable to an Empty Object in TypeScript # Use type assertions to initialize a typed variable to an empty object, e.g. const a1 = {} as Animal; . You can then set the properties on the object using dot or bracket notation. All of the properties you set on the object need to conform to the type.
To declare an empty array in Java, we can use the new keyword. After the declaration of an empty array, we can initialize it using different ways. The syntax of declaring an empty array is as follows. data-type[] array-name = new data-type[size]; //or data-type array-name[] = new data-type[size];
To declare an array of objects in TypeScript, set the type of the variable to {}[] , e.g. const arr: { name: string; age: number }[] = [] . Once the type is set, the array can only contain objects that conform to the specified type, otherwise the type checker throws an error. Copied!
The existing answers missed an option, so here's a complete list:
// 1. Explicitly declare the type
var arr: Criminal[] = [];
// 2. Via type assertion
var arr = <Criminal[]>[];
var arr = [] as Criminal[];
// 3. Using the Array constructor
var arr = new Array<Criminal>();
Explicitly specifying the type is the general solution for whenever type inference fails for a variable declaration.
The advantage of using a type assertion (sometimes called a cast, but it's not really a cast in TypeScript) works for any expression, so it can be used even when no variable is declared. There are two syntaxes for type assertions, but only the latter will work in combination with JSX if you care about that.
Using the Array constructor is something that will only help you in this specific use case, but which I personally find the most readable. However, there is a slight performance impact at runtime*. Also, if someone were crazy enough to redefine the Array constructor, the meaning could change.
It's a matter of personal preference, but I find the third option the most readable. In the vast majority of cases the mentioned downsides would be negligible and readability is the most important factor.
*: Fun fact; at the time of writing the performance difference was 60% in Chrome, while in Firefox there was no measurable performance difference.
The issue of correctly pre-allocating a typed array in TypeScript was somewhat obscured for due to the array literal syntax, so it wasn't as intuitive as I first thought.
The correct way would be
var arr : Criminal[] = [];
This will give you a correctly typed, empty array stored in the variable 'arr'
Hope this helps others!
I know this is an old question but I recently faced a similar issue which couldn't be solved by this way, as I had to return an empty array of a specific type.
I had
return [];
where [] was Criminal[] type.
Neither return: Criminal[] []; nor return []: Criminal[]; worked for me.
At first glance I solved it by creating a typed variable (as you correctly reported) just before returning it, but (I don't know how JavaScript engines work) it may create overhead and it's less readable.
For thoroughness I'll report this solution in my answer too:
let temp: Criminal[] = [];
return temp;
Eventually I found TypeScript type casting, which allowed me to solve the problem in a more concise and readable (and maybe efficient) way:
return <Criminal[]>[];
Hope this will help future readers!
For publicly access use like below:
public arr: Criminal[] = [];
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