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JavaScript, like any other programming language, has its own data structures and types.
JavaScript has a few data types that we have to know, to build programs with it. Different pieces of data can be put together to build more complex data structures.
JavaScript is a loosely typed, or dynamically typed, language. This means that a variable that’s declared with one type can be converted to another type without explicitly converting the data to another type.
Variables can also contain any type at any time, depending on what’s assigned. With dynamically typed languages, it’s hard to determine the type that a variable has without logging it and we might assign data that we don’t want in the variable.
TypeScript rectifies these issues by letting us set fixed types for variables so that we’re sure of the types. In this article, we’ll look at the void, null, undefined, never, and the object types.
Void
The void type is pretty much the opposite of the any type. It means the absence of any type. So, the variable of the void type can only have the value null if the --strictNullChecks setting isn’t specified when running the TypeScrip compiler or it can be set to undefined.
Therefore, directly assigning values to a void variable isn’t very useful. It’s more used for specifying the return data type of a function. A function that has the void return type doesn’t return anything.
For example, we can do a useless assignment to a void variable like in the following code:
let useless: void = undefined;
We can set the return type of an arrow function to void by writing:
const voidFn = (): void => {
console.log("Void function returns nothing");
}
voidFn();
Alternatively, we can set the return type to void for a traditional function like in the following code:
function voidFn(): void {
console.log("Void function returns nothing");
}
voidFn();
Both function calls will output Void function returns nothing from the console.log statement inside the function.
Null
The null type represents a variable that can only take on the value null. null means that there's no value for a variable.
So, once again, assigning a value to it is pretty useless. We can only assign null to a variable that has the variable null. With the --strictNullChecks flag set when running the TypeScript compiler, null can only be assignable to variables with the any type and the null type.
Where it does become useful is that we can have variables that can have values from more than one assigned to it with union types.
Then, we can specify something useful along with the null type. This is different from JavaScript in that the value null is the type object instead of null for historical reasons.
Undefined
The undefined type represents a variable that can only take on the value undefined. So, once again, assigning a value to it is pretty useless.
We can only assign undefined to a variable that has the variable null. undefined can only be assignable to variables with the any type and the undefined type.
Where it does become useful is that we can have variables that can have values from more than one assigned to it with union types.
Then, we can specify something useful along with the undefined type. It’s practically useless on its own, so we shouldn’t see many cases with variables that have only the undefined type.
Never
The never type is a type of value that represents something that never occurs. It’s like void in that it’s useful for designating that a function never returns anything.
The never type is a sub-type of, and is assignable to, every type. However, no type is a sub-type of, or assignable to, the never type except for other never variables.
A function that has a never return type must always have an unreachable endpoint. For example, we can write a function that has an infinite loop that has the never return type like in the following code:
function infiniteFn(): never {
while (true) {
}
}
A function that throws an exception may also have the never return type, like in the following example:
function errorFn(message: string): never {
throw new Error(message);
}
errorFn('Error occurred');
Object
The object type is a type that represents non-primitive objects. That is, anything that’s not a number, string, boolean, bigint, symbol, null, or undefined.
It’s mainly used in the type definition of the Object object in the standard library and other pieces of code that don’t want primitive values to be assigned to it, or passed into a function.
For example, in the type definition of the Object.create method, we see that the type of the parameter is set to the object like in the following code:
create(o: object | null): any;
Likewise, in the signature of the setPrototypeOf method in the same type declaration, we see that the proto parameter, which is the parameter that takes the prototype of an object, also has the object type set, as it does below:
setPrototypeOf(o: any, proto: object | null): any;
This way, we can’t pass in primitive values into these methods as arguments in TypeScript. If we did, then we would get an error. So, if we have the following code in our TypeScript files, then they would be compiled and run:
const obj1 = Object.create({});
const obj2 = Object.create(null);
console.log(obj1);
console.log(obj2);
We would get an empty object in both console.log statements.
The only difference is that obj2 is a pure object, which means it doesn’t inherit from any prototype. Passing in primitive values to the create method like in the code below will cause compilation to fail:
Object.create(42);
Object.create("abc");
Object.create(false);
Object.create(undefined)
The code above would get us an Argument of type ‘42’ is not assignable to parameter of type ‘object | null’ error for the first one, Argument of type ‘abc’ is not assignable to parameter of type ‘object | null’ for the second one.
The third line would get us Argument of type ‘false’ is not assignable to parameter of type ‘object | null’, and the last line would get us Argument of type ‘undefined’ is not assignable to parameter of type ‘object | null’.
Compilation would fail and the code wouldn’t run.
Also, we can use it to prevent primitive values from being assigned to it. For example, if we write:
let x: object = {};
Then, the code above would be compiled and run. However, if we write the following instead:
let x: object = 1;
Then we get Type ‘1’ is not assignable to type ‘object’ and the code can’t be compiled with the TypeScript compiler and be run, since 1 is a primitive value.
The void type is pretty much the opposite of the any type. It means the absence of any type. So, the variable of the void type can only have the value null if the --strictNullChecks setting isn’t specified when running the TypeScrip compiler or it can be set to undefined.
It’s useful for declaring functions that don’t return anything and not much else. The null type variable can only take on the null value. The undefined type can only be assigned the value undefined.
The object type is a type that represents non-primitive objects. That is, anything that’s not a number, string, boolean, bigint, symbol, null, or undefined.
It’s mainly used in the type definition of the Object object in the standard library and other pieces of code that don’t want primitive values to be assigned to it, or passed into a function.
