Refactoring JavaScript by Evan Burchard

Author:Evan Burchard
Language: eng
Format: epub, mobi, pdf
Publisher: O'Reilly Media
Published: 2017-03-28T04:00:00+00:00

this.labelProbabilities.forEach((_probabilities, difficulty) => { ... const totalLikelihood = chords.reduce((total, chord) => {

The parts with => are what make them arrow functions. The syntax might seem a little weird (and it has a lot of variation), but what’s great is the this passes through from the outer function.

To simplify this function a bit further, smoothing can be taken out of the function and added as a new attribute of the classifier:

smoothing: 1.01, classify: function(chords){ const classified = new Map(); this.labelProbabilities.forEach((_probabilities, difficulty) => { const totalLikelihood = chords.reduce((total, chord) => { const probabilityOfChordInLabel = this.probabilityOfChordsInLabels.get(difficulty)[chord]; if(probabilityOfChordInLabel){ return total * (probabilityOfChordInLabel + this.smoothing); }else{ return total; } }, this.labelProbabilities.get(difficulty) + this.smoothing); classified.set(difficulty, totalLikelihood); }); return classified; }

On the one hand, this is nice because it gets smoothing out of our function. However, it does increase the scope of where it is available and necessitates adding this. to precede it inside of the function. Extracting this const to an attribute also makes it assignable again. Unfortunately, getting something like a const inside of our classifier object literal takes a bit of work. There is a defineProperty function that we can use to define attributes that are not only not writable, but even immutable by default. We won’t go into depth here, but defineProperty is a good tool to look into if you want a lot of control over how properties are used.

Normally, defineProperty would be called after the object is created, which would add complexity to our execution at some point, and move the attribute’s creation to some other physical place in the file. On the other hand, we could just call this inside of the classify function like this:

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