How to do currying with UnderscoreJS?

Question

I am a bit experimenting with _.bind(...). I see how to force a function context with bind, but don't yet see how to do currying.

What I try is this:

 add = function(number) { this.sum = this.sum + number; }
 add5 = _.bind(add, { sum: 0 }, 5)

However, calling add5(), or add5(5) seems not to have some effects.

Any clues how to do wrap the arguments such that the context is preserved from one call to another?

Answer 1

Probably you want to do partial application, not currying/schönfinkeling. Underscore has the _.partial function for this:

function add(a, b) { return a+b; }
var add5 = _.partial(add, 5);

You can as well use _.bind, and it has some effects. For example:

var add5 = _.bind(add, null /*context is irrelevant*/, 5);
add5(3); // returns 8

Yet, your function did not return anything, and the context which you did change was not accessible. However:

var ctx1 = {sum: 0};
function add(a) { this.sum += a; } // returns nothing!
var addto1 = _.bind(add, ctx1);
addto1(5); // undefined
ctx1; // {sum: 5}

var add5to1 = _.bind(add, ctx1, 5);
add5to1(); // undefined
ctx1; // {sum: 10}

var ctx2 = {sum: 5};
add3to2 = _.bind(add, ctx2, 3);
add3to2(); // undefined
ctx2; // {sum: 8}