How to use JavaScript functions as values and parameters?

In javascript function are first class objects. That means it can be used as values. Notice the example below

// function definition
function alertHi(name) {
   alert('hi! '+name);
}

// storing string
var name = 'rajakvk';

// storing number
var age = 26;

// storing function
var sayHi = alertHi;

// now sayHi is also function, you can call it like
sayHi();   // hi

Here is example to use function as parameter

// define a function to call a function twice
// here function f passed as parameter
var callTwice = function(f,x) {
   return f(f(x));
}
var addTen = function(x) {
   return x+10;
}
// here function addTen used as value 
var y = callTwice(addTen, 2);   // 22

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