TypeScript Union

The TypeScript union has the ability to combine one or two different types of data (i.e., number, string, float, double, etc). It is the most powerful way to express a variable with multiple types. Use pipe (‘|’) symbol to combine two or more data types to achieve Union type.

Syntax:

(type1|type2|type3|...|type-n)

Example:

let value: number | string;
value = 190;
console.log("Numeric value of the value: " + value);
value = "Welcome to TypeScript!";
console.log("String value of the value: " + value);

Compiling the above code to generate the following JavaScript code.

let value: number | string;
value = 190;
console.log("Numeric value of the value: "+value);
value = "Welcome to TypeScript!";
console.log("String value of the value: "+value);

Output:

190
Welcome to TypeScript!

Example: In this example the geeks is of union type, denoted using (string | number). So, we can assign a string or a number to it nothing else is allowed.

let geeks: (string | number);
geeks = 123;   // OK
geeks = "XYZ"; // OK
geeks = true;  // Compiler Error

Function Parameter as Union Type: We can pass the function as a parameter. In this example, parameter geeks is of union type. You can pass either a string value or a number value otherwise the compiler will give an error.
Example:

function displayType(geeks: (string | number)) {
    if(typeof(geeks) === "number")
        console.log('geeks is number.')
    else if(typeof(geeks) === "string")
        console.log('geeks is string.')
}

// Output: Code is number.
displayType(49);

// Output: Code is string.
displayType("GFG");

// Compiler Error: Argument of type 'true' is not
// assignable to a parameter of type string | number
displayType(true);

Array as Union Type: In union type we can also pass an array. The program declares an array. The array can represent either a numeric collection or a string collection.
Example:

//Generated by typescript 1.8.10
var arr = [2, 5, 7, 5, 11, 15];

console.log("Display the array elements");

// Loop to display array elements
for (var i = 0; i < arr.length; i++) {
   console.log(arr[i]);
}

// Declare another array
arr = ["Geeks", "G4G", "GFG", "GeeksforGeeks"];

console.log("Display the array elements");

// Loop to display the array elements
for (var i = 0; i < arr.length; i++) {
   console.log(arr[i]);
}

Output:

Display the array elements
2
5
7
5
11
15
Display the array elements
Geeks
G4G
GFG
GeeksforGeeks

Union can Replace enums: It is a list of constant types that is created by Enums. By default, enums have index values (0, 1 ,2, 3, etc). Enums are actually transpiled (taking source code written in one language and transforming into another language that has a similar level of abstraction) and end up into the result like JavaScript.