TypeScript: Modern JavaScript Development

Functional programming

When you ask a developer what the definition of a function is, he would probably answer something like "something that does something with some arguments". Mathematicians have a formal definition for a function:

A function is a relation where an input has exactly one output.

This means that a function should always return the same output for the same input. Functional programming (FP) uses this mathematical definition. The following code would violate this definition:

let x = 1; 
function f(y: number) { 
  return x + y; 
} 
 
f(1); 
x = 2; 
f(1); 

The first call to f would return 2, but the second would return 3. This is caused by the assignment to x, which is called a side effect. A reassignment to a variable or a property is called ...