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5 things I learnt during my latest Javascript Code Kata

Erik Pischel on August 30, 2018

Sometimes I do a code kata at codewars.com. That is a fun way to solve computer science related problems, learn on the way to solve them and especi...

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Ingus Mat Burleson • Aug 31 '18

Don't forget that while you can't redeclare variables using let, you can reassign them.

let a = 42;
console.log(a); // 42
let a = 24; // SyntaxError: redeclaration of let a
a = 24;
console.log(a) // 24

If you want a var that can not be reassigned, use const instead of let.

const c = 43;
console.log(c) // 43
c = 51; // TypeError: invalid assignment to const `c'

Viktor Shyrokov • Aug 31 '18 • Edited

Just interesting, we have such example

let set = {};
edges.filter(e => e[0][1] !== e[0][0]).forEach(e => { if (!set[e[0]] || minOrMaxFunc(set[e[0]], e[1])>00) { set[e[0]] = e[1]; } });

let's say all the records match filter, in this case we will loop through all the edges twice?

Maybe using filter with forEach not the best example, what do you think about that?

Erik Pischel • Aug 31 '18

Why do you think it will loop twice?

Context: input in this case is a weighted graph like this [["AB", 4], ["BC", 8], ["AC", 5]]. A, B, C are nodes, numbers are weights.

It is just a way to initialize a data structure. filter is used just to avoid "saving" edges like ["AA", 8] in object "set".

Viktor Shyrokov • Sep 2 '18

It could loop twice, let's say we have 1 billion of records.

All that records, as example has prop like, isFilterable = true

let's say we have a filter

edges.filter(e => e.isFilterable).forEach(e => { // some logic });

in this case we will loop through all the data twice.

It would be easier to use such a condition in the foreEach directly.

In this case you'll avoid duplicate actions on data.

Yes, filter pretty cool thing, but not in this case where we have filter().foreach()

Erik Pischel • Sep 5 '18

Now I see your point. The advantage I see in this case is that a seperate "filter" step represents a seperate step in the algorithm. Like "step 1: filter out all edges that connect only one node. step 2: ...". If you use the filter condition in the forEach directly it gets more entangled with the next step.

Thanks for your comment.

Sylvain Lesage • Sep 1 '18

I really don't understand this part of code. Is something missing?

// sort edges by weight (min/max)
const asc = t === "min";
edges.sort(([edge_a, a], [edge_b, b]) => asc ? a - b : b - a);

Erik Pischel • Sep 1 '18

Well, "t" is either "min" or "max". And edges is an array of 2 element arrays, e.g.

edge=[["AB",3], ["BC",9], ["AC",5]]

sort takes a function of two parameters that returns a number indicating whether the first parameter is smaller or bigger than the second parameter. The parameters are elements of the array. In our case these are 2 element arrays. In the code example, these are destructured as edge_a (e.g. "AB") and a (e.g. 3) respectively edge_b and b. The array edges is being sorted by the second element of the 2 element arrays.
Hope this helps.

Sylvain Lesage • Sep 1 '18 • Edited

Ah OK. I didn't understand that t was a variable previously set to "min" or "max". Maybe it would be clearer to precise it, o add a line of code to show it.

Thanks for your article, btw.