What do you call a function that takes a value and returns a function that returns that value?

Question

I am trying out a more functional style with Javascript using Lodash and compose. I notice that I sometimes need a function that returns a value. So was wondering what this was called so I can find out if Lodash actually has this method.

var returnFn = function (i) {
  return function () {
    return i;
  };
};

Example:

_.compose(doSomething, returnFn({ foo: 'bar' });

Instead of:

_.compose(doSomething, function () {
    return { foo: 'bar' };
});

Answer 1

What do you call a function that takes a value and returns a function that returns that value?

What you have seems to be a curryied identity function, which in Haskell is known as const. As it returns a function, it is a higher order function, maybe also called function factory. The returned function - a constant function - is a closure as it has access to the outer function's argument.

so I can find out if Lodash actually has this method.

No, it doesn't. However, you can easily compose it yourself by _.binding _.partial to the _.identity function:

var returnFn = _.bind(_.partial, null, _.identity);

Your plain implementation will be lots of faster though…

_.compose(doSomething, returnFn({ foo: 'bar' }));

What you're doing there is just partial application, and you should not use compose for it. Go with

_.partial(doSomething, {foo: 'bar'});

Answer 2

They are called higher order functions.