TypeScript Union Types

Union Types allow variables to be set to multiple types using the pipe (|) operator. The variable can then be assigned values based on the specified types.

Note: Only the specified types can be assigned; assigning other types will result in an error.

The syntax for creating a union type is as follows:

Type1|Type2|Type3

Example

Declare a union type:

TypeScript

var val: string|number 
val = 12 
console.log("Number is " + val) 
val = "tutorialpro" 
console.log("String is " + val)

Compiling the above code yields the following JavaScript code:

JavaScript

var val;
val = 12;
console.log("Number is " + val);
val = "tutorialpro";
console.log("String is " + val);

The output is:

Number is 12
String is tutorialpro

Assigning a different type will result in an error:

var val: string|number 
val = true

Union types can also be used as function parameters:

TypeScript

function disp(name: string|string[]) { 
        if (typeof name == "string") { 
                console.log(name) 
        } else { 
                var i; 
                for (i = 0; i < name.length; i++) { 
                console.log(name[i])
                } 
        } 
} 
disp("tutorialpro") 
console.log("Output array....") 
disp(["tutorialpro", "Google", "Taobao", "Facebook"])

Compiling the above code yields the following JavaScript code:

JavaScript

function disp(name) {
        if (typeof name == "string") {
                console.log(name);
        }
        else {
                var i;
                for (i = 0; i < name.length; i++) {
                console.log(name[i]);
                }
        }
}
disp("tutorialpro");
console.log("Output array....");
disp(["tutorialpro", "Google", "Taobao", "Facebook"]);

The output is:

tutorialpro
Output array....
tutorialpro
Google
Taobao
Facebook

Union Type Arrays

Arrays can also be declared as union types:

TypeScript

var arr: number[]|string[]; 
var i: number; 
arr = [1, 2, 4] 
console.log("**Number Array**")  

for (i = 0; i < arr.length; i++) { 
   console.log(arr[i]) 
}  

arr = ["tutorialpro", "Google", "Taobao"] 
console.log("**String Array**")  

for (i = 0; i < arr.length; i++) { 
   console.log(arr[i]) 
}

Compiling the above code yields the following JavaScript code:

JavaScript

var arr;
var i;
arr = [1, 2, 4];
console.log("**Number Array**");
for (i = 0; i < arr.length; i++) {
    console.log(arr[i]);
}
arr = ["tutorialpro", "Google", "Taobao"];
console.log("**String Array**");
for (i = 0; i < arr.length; i++) {
    console.log(arr[i]);
}

The output is:

**Number Array**
1
2
4
**String Array**
tutorialpro
Google
Taobao