What is a TypeScript union?
Overview
In TypeScript, we need to specify data types for variables.
let a:string;
We might come across a situation where the data can be of different data types, depending upon the situation.
For example, if we want to store string and boolean in variable a, then we use TypeScript union.
-
You can combine different data types.
-
Use the pipe
|operator to separate data types.
Syntax
let variableName:datatype 1| datatype 2 | ... | datatype n
Code
Example 1
In the code snippet below:
-
We create variable
awith typesstringandboolean. -
In line 2, we assign the value
John, which is of string type, toa. -
In line 3, we assign
true, which is ofbooleantype, toa.
let a:string | boolean;a = "John"console.log(a)a = trueconsole.log(a)
Example 2
The compiler will throw an error if we assign values of types other than the specified data types.
In the following code snippet, we assign value 1, which is of number type, to a.
- The compiler will throw the error
Type '1' is not assignable to type 'string | boolean'.
let a:string | boolean;a = "John"a = true//will throw errora = 1
Example 3
We can create custom types using TypeScript unions and type keywords so that we can assign new custom types to different variables.
In the code snippet below:
-
We create a
customTypethat represents data typesstring,number, andboolean. -
In lines 3 and 4, we create variables
aandbwith the typecustomType.
// creating custom type usign uniontype customType = (string | number | boolean);//assigning custom typelet a:customType;let b:customType;a = 5console.log(a)a = "John"console.log(a)a = trueconsole.log(a)b = 10console.log(b)
