Explanation of Following Javascript Code

Question

I am fairly new to js and I have been looking at the Mozilla Developer site. Under the functions section, I can't seem to grasp the following

function map(f,a) {
  var result = [], // Create a new Array
      i;
  for (i = 0; i != a.length; i++)
    result[i] = f(a[i]);
  return result;
}

particulary, this line "result[i] = f(a[i]);"

From Mozilla: Function expressions are convenient when passing a function as an argument to another function. The following example shows a map function being defined and then called with an anonymous function as its first parameter

Can you help explain this?

Here is a link for reference. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Functions

stackoverflow.com/questions/1140089/…

user2591612
– user2591612

2014-02-03 15:35:33 +00:00
Commented Feb 3, 2014 at 15:35 — user2591612
– user2591612, Commented Feb 3, 2014 at 15:35

smnbbrv · Accepted Answer · 2014-02-03 15:45:01Z

0

this piece of code applies to every element of array "a" function "f" and returns the array "result" which contains the results of running function "f" for every element in "a".

Sorry, I've got a recursion :)

Actually it can be used like that:

var array = map(function(val){ return val + 1; },[1,2,3,4,5]);

and in "array" you will have these values:

[2,3,4,5,6]

edited Feb 3, 2014 at 15:45

answered Feb 3, 2014 at 15:34

smnbbrv

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7 Comments

Loïc Faure-Lacroix Over a year ago

There is no recursion there. Unless you pass map as f.

smnbbrv Over a year ago

no, the recursion is in my mind :) in the words I've written. In the function there is no recursion at all. Just a joke

user3259232 Over a year ago

But how is "f" a function here when to me it looks like it's a parameter of the map function?

Loïc Faure-Lacroix Over a year ago

@user3259232 you can send function as parameter of function.

smnbbrv Over a year ago

yes, exactly. You can create variables with type function in JS and pass them as parameters

|

Jeremy Quinton · Accepted Answer · 2014-02-03 15:47:31Z

-1

result[i] = f(a[i]);

result is an array and the element at index i is being assigned the result of a function call which takes the parameter a[i].

f in the line above is an anonymous function which is parsed into the map function as the first argument. f would be defined somewhere else in code with the following syntax

function(value) {
    return result. 
}

The calling code of your example above would look something like.

var array = map(function(value) { return result; },[1,2,3,4,5]);

I would read up on anonymous functions here which might help. http://en.wikibooks.org/wiki/JavaScript/Anonymous_Functions

edited Feb 3, 2014 at 15:47

answered Feb 3, 2014 at 15:42

Jeremy Quinton

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