Function type interfaces

We just saw how to define properties in interfaces. Similarly, we can also define function types in interfaces. We can define function types in interfaces by just giving the signature of the function with the return type. Note that, in the following code snippet, we have not added the function name:

interface AddCustomerFunc { 
  (firstName: string, lastName: string): string; 
} 

Now, we have AddCustomerFunc ready. Let's define an interface variable called AddCustomerFunc and assign a function of the same signature to it as follows:

var addCustomer: AddCustomerFunc; 
addCustomer = function(firstName: string, lastName: string) { 
  console.log('Full Name: ' + firstName + ' ' + lastName); 
  return firstName + ' ' + lastName; 
} 

The parameter name in the function signature can vary, but not the data type. For example, we can alter the fn and ln function parameters of the string type as follows:

addCustomer = function(fn: string, ln: string) { 
  console.log('Full Name: ' + fn + ' ' + ln); 
} 

So, if we change the data type of the parameter or the return type of the function here, the compiler will throw an error about the parameter not matching or the return type not matching with the AddCustomerFunc interface.