What are type aliases in TypeScript?
Overview
In TypeScript, type aliases are used to assign a name for any type. It doesn't create a new type. Instead, it provides a name for the type.
Create type alias
To create a type alias, we need to use the type keyword.
type gender = "male" | "female" | "nonbinary";let male:gender = "male";let female:gender = "female";let nonbinary:gender = "nonbinary";console.log(male, female, nonbinary)
In the code above, we have created a type alias with the name gender. The gender type can have one of the following values: "male", "female", or "nonbinary".
When we try to assign a value other than the configured value, we get a compilation error. The code widget below demonstrates this case:
type gender = "male" | "female" | "nonbinary";// assigning value which is not configured will throw compilation errorlet err:gender = "gen";
Examples
type alphaNumeric = string | number;let num:alphaNumeric = 10;let str:alphaNumeric = "ten";type Point = {x: number;y: number;};let pt:Point = {x: 10, y: 20};// obj type variable can have any one of the below valuetype obj = {val: 1} | {val: 2};// obj type variable can have any one of the below valuetype func = (() => string) | (() => void);
Explanation
- Line 1: We create a type alias with the name
alphaNumeric. ThealphaNumerictype can have either a string or a numeric value. - Lines 6–9: We create a type alias with the name
Point. ThePointtype is an object that has two numeric properties,xandy. - Line 14: We create a type alias with the name
obj. The value of theobjtype should either be{val: 1}or{val: 2}. Similarly, we create a type forfuncin line 17.
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