3.8 Currying & Bound Methods

>>>

function mul(a, b) {
    return a * b;
}

Let's first create a simple function to multiply two values.

>>>

var factorThree = mul.bind(null, 3);

Using the built-in bind() method for function objects, we can create a new function where one or more of the arguments are already set. This is also called currying and is another way to create higher-order functions.

>>>

factorThree(2)

When calling the new function, the original function is called with both the supplied argument and the bound one.

>>>

function factor(a) {
    return mul.bind(null, a);
}

With the help of bind(), it is now possible to re-implement the factor() function from the previous page.

>>>

[0, 1.5, -3].map(factor(3))

[0, 4.5, -9]

Using the built-in map() function for arrays also shortens the previous example code.

>>>

var arrMap = [].map.bind([0, 1.5, -3])

The first argument to the bind() method is the object that the function is bound to (as a method). So here we create a version of map() that is bound to a particular array.

>>>

arrMap(factor(3))

[0, 4.5, -9]

This way of creating functions bound to an object is actually very useful when mixing object-oriented programming and functional programming styles in JavaScript.

>>>

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