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JavaScript Patterns: Callback

In JavaScript, functions are objects that can be passed as arguments to other functions. A useful pattern that lets you take advantage of this is the Callback Pattern.

For example, let's say you were implementing a graphing library that would allow you to draw column and line charts. One way of writing it might be something like this:

The problem with this code is that the logic for calculating the x and y positions is duplicated in each function. Extracting this to another function is problematic because it may need all of the values that you are trying to plot in order to calculate the positions. One way around this might be to pass in all the values, create an array of all the and y coordinates and then draw those. However, this requires looping twice, once over the values themselves, and again over all the calculated coordinates.

This problem can be solved more efficiently by using a callback like this:

Here we've extracted the positioning calculations into a separate plot function. This function takes an array of values to plot as well as a function. When the plot function is called, we iterate over the values and calculate the x and y positions. We then pass the x and y coordinates as well as the actual value to the drawingCallback function which handles the actual drawing. This way we're able to keep our code DRY and efficient.



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