Functions in JS

A function definition is a regular binding where the value of the binding is a function.For example,

const square = function(x) {
return x * x;
};

• Each function will have both a definition and declaration.
  • Declaration is basically you simply telling JS to make a particular function, and witholding the info on how the function will process your arguments.

function declare();

• Definition on the other hand, will tell JS exactly what your function will do with the input arguments.

function declare(){
console.log("Hi, I am Param");
}

• Scope : Each variable will have a scope, usually global or local.
  • Global scope indicates that the variable can be used by any function or block of code in our program.
  • Local scope means that the variable is valid only within our code block, for example, inside a function. No code segment outside our function(in the above example) will be able to access a local variable.
  • Note : Nesting of functions is possible, i.e., you can declare functions within functions, code blocks within other code blocks. This means that we can have a lot of local variables and global variable pairs too.

The arrow notation :

• Quoting from EloquentJS, "There’s a notation for functions, which looks very different from the others. Instead of the function keyword, it uses an arrow (=>) ."

• Its syntax goes like :

  const power = (base, exponent) => {
let result = 1;
for (let count = 0; count < exponent; count++) {
result *= base;
}
return result;
};

  • The arrow comes after all the function arguments, and is followed by the normal function body, as in the usual syntax that we have seen above.

Optional arguments

• A function can be given additional arguments, in the sense you can provide 3 arguments to it, when it has only one argument. This is perfectly fine, and JS will ignore you and do your work.
• Also, you can make define a function as

const bio = (name, age = 25) =>{ //age is an optional argument
console.log("Name :" + name + " Age :" + age);
}
//call 1
bio("param");
//call 2
bio("param",19);
//output 1 : Name :param Age : 25
//output 2 : Name :param Age : 19

In the above function, both calls are perfectly valid. 

Recursion

As stated above, you can have functions calling other functions, and even itself within the function. This way of calling a function within itself is known as recursion.

For recursion, the function will have 2 main things :
* A base condition, on which it will exit the recursive call
* The call to function itself
* Optional : Any other logic/programming, if required

function power(base, exponent) {
if (exponent == 0) {
return 1;
} else {
return base * power(base, exponent - 1);
}
}
console.log(power(2, 3));
// → 8

Review

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