TypeScript Union
A TypeScript Union variable can hold a value that belongs to one of the many declared datatypes.
Syntax of TypeScript Union
To specify the different datatypes that a union variable can hold, use the following syntax:
var varname: datatype1|datatype2|datatype3varis the keywordvarnameis the name of the union variabledatatype1, datatype2, . .datatypes that the variable can hold should be separated by vertical bar
Example
Following is an example of TypeScript Union, where a variable can hold data of either number or string.
example.ts
var ns : number|string
ns = 25
console.log("type of ns is : "+(typeof ns))
console.log("value of ns : "+ns)
ns = "Twenty Five"
console.log("\ntype of ns is : "+(typeof ns))
console.log("value of ns : "+ns)On traspiling the code, following JavaScript is generated.
example.js
var ns;
ns = 25;
console.log("type of ns is : " + (typeof ns));
console.log("value of ns : " + ns);
ns = "Twenty Five";
console.log("\ntype of ns is : " + (typeof ns));
console.log("value of ns : " + ns);
Output
type of ns is : number
value of ns : 25
type of ns is : string
value of ns : Twenty FiveUnion Type with Arrays
Even TypeScript Array can be declared as union type variable, i.e., the array variable can hold array of elements that belong to any a given set of datatypes.
Example
In the following example, variable b is declared as a union type variable that can hold either a number or number array.
example.ts
var a : number
var b : number|number[]
a = 10
b = 5
var c = a + b
console.log("sum of a and b : "+c)
b = [4, 8, 9, 1, 5, 6]
var c = a
b.forEach(i => {
c = c+i
})
console.log("sum of a and b : "+c)When traspiled to JavaScript, following .js file is generated.
example.js
var a;
var b;
a = 10;
b = 5;
var c = a + b;
console.log("sum of a and b : " + c);
b = [4, 8, 9, 1, 5, 6];
var c = a;
b.forEach(function (i) {
c = c + i;
});
console.log("sum of a and b : " + c);
Output
sum of a and b : 15
sum of a and b : 43Conclusion
In this TypeScript Tutorial – TypeScript Union, we have learnt how to declare a variable as union type. We also looked into the declaration of array variables as union type.