String Literal Types

In TypeScript, string literals allow you to specify the exact value a string must have in it’s lifespan. You can assume it a form as ‘string based enum’ which also is named group of string constants.

Syntax

Use ‘pipe’ symbol between different allowed string values.

type myVar = "value1" | "value2" | "value3" | "value4";	//upto N values

//For example

type AppStatus = "ACTIVE" | "INACTIVE" | "ONHOLD";		

String Literal types example

Let’s see how we can use string literal and how we cannot.

1. Variable Assignment

   You can assign only allowed values to literal type variable. Else it will be compile time error.

   type AppStatus = "ACTIVE" | "INACTIVE" | "ONHOLD";
   
   let currStatus: AppStatus;
   
   currStatus = "ACTIVE";		//OK
   
   currStatus = "DELETED";		//Error - Type '"DELETED"' is not 
   							//assignable to type 'AppStatus'
   

2. Function Parameter

   You can pass only allowed values to literal type argument. Else it will be compile time error.

   type AppStatus = "ACTIVE" | "INACTIVE" | "ONHOLD";
   
   function showMe(currentStatus: AppStatus): void {
   	console.log(currentStatus);
   }
   
   showMe('ACTIVE');	//OK - Print 'ACTIVE'
   
   showMe('DELETED');	//Compile time Error
   

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Happy Learning !!