Union in Typescript

TypeScript 1.4 gives programs the ability to combine one or two types. Union types are a powerful way to express a value that can be one of the several types. Two or more data types are combined using the pipe symbol (|) to denote a Union Type. In other words, a union type is written as a sequence of types separated by vertical bars.

Syntax: Union literal

Type1|Type2|Type3 

Example: Union Type Variable

var val:string|number 
val = 12 
console.log("numeric value of val "+val) 
val = "This is a string" 
console.log("string value of val "+val)

In the above example, the variable’s type is union. It means that the variable can contain either a number or a string as its value.

On compiling, it will generate following JavaScript code.

//Generated by typescript 1.8.10
var val;
val = 12;
console.log("numeric value of val " + val);
val = "This is a string";
console.log("string value of val " + val);

Its output is as follows −

numeric value of val  12 
string value of val this is a string 

Example: Union Type and function parameter

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("mark") 
console.log("Printing names array....") 
disp(["Mark","Tom","Mary","John"])