29 January 2014

I appreciate orthogonality in programming. If you name a method in your app “out,” I might expect its pair-opposite, “in” to appear. Even more interesting, I enjoy seeing the implementation of “in” referencing “out” in some way (or visa versa).

While reading docs for various “functional” JavaScript libraries, I noticed most have a take method. But I can’t find a leave method anywhere. So, here’s an introduction to leave, what it does, why you need it, and why it balances take.

In a classic JavaScript scenario, the n00b functional programmer reuses the uber-useful function sum in a map instead of add because he wants to bind a couple of other parameters.

function add( a, b ) {
return a + b;

function sum() { //variadic
return Array.prototype.slice.call( arguments, 0 ).reduce( add, 0 );

[1, 2].map( sum.bind( null, 1, 2 ) );
//["41,2", "61,2"] (╯°□°)╯︵ dɐɯ

The poor n00b expected an array [4, 5] but instead created all kinds of errors with ["41,2", "61,2"] and can’t figure out why.

take, in JavaScript, works like Array.prototype.slice but always starting at the 0 index of an array. So, list.take( x ) should have the same output as list.slice( 0, x ).

We can use leave to “guard” the parameters passed from map. leave will only leave one spot open to the signature of sum.

//n00b uses _part_, it's super effective!
"reduce", "slice",
].forEach( _part_._borrow( this, Array.prototype ) );

var take_ = slice_.bind( null, 0 );

function add( a, b ) {
return a + b;

function sum() {
return _reduce( arguments )( add, 0 );

function leave( n ) {
return function ( fn ) {
return function () {
return fn.apply( this, take_( n )( arguments ) );

[1, 2].map( leave( 1 )( sum.bind( null, 1, 2 ) ) );
//[4, 5]

For fun, click the “repl” link below and change the code to ...leave( 2 ).... Why did that happen?

Try it in the repl


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