Compose With Reduceright
29 January 2014
Poor .reduceRight.
This JavaScript late-comer (ES 5.1) does the same thing as .reduce, only backwards. No wonder this seemingly
forgotten function gets no love.
Well, I love you, .reduceRight. With the help of _part_, I made
a compose function which I find more elegant than my previous versions of composition.
_part_._borrow( this, Array.prototype )( "reduceRight" );
function csub( g, f ) {
return f( g );
}
var _compose = _part_._create( function ( x ) {
return reduceRight_( csub )( this , x );
} );csub represents the composition operator.
So, you might read it aloud as “c sub g f” which equates to f(g). But that’s just academic stuffs.
The cool part happens when I make a slight change to an example from lodash:
//this seems weird, but order matters
//flip the compose fn's below to see how
var realNameMap = {
'pebbles': 'penelope'
};
var format = function(name) {
name = realNameMap[name.toLowerCase()] || name;
return name.charAt(0).toUpperCase() + name.slice(1).toLowerCase();
};
var greet = function(formatted) {
return 'Hiya ' + formatted + '!';
};
//I now prefer arrays of "un-composed" functions over
//only having composed groups as arguments or needing to .apply the array
var welcome = _compose( [greet, format] );
welcome('pebbles');
// → 'Hiya Penelope!'This makes me even more excited for the upcoming rest parameters and spread operator because I can use them to collect extra arguments for the first invocation with one small change:
var _compose = _part_._create( function ( ...x ) {
return reduceRight_( csub )( this , ...x );
} );