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What are type aliases in TypeScript?
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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)
Creating a type alias
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 error let err:gender = "gen";
Type alias error
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 value
type obj = {val: 1} | {val: 2};
// obj type variable can have any one of the below value
type func = (() => string) | (() => void);Type alias example
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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