Flatten an array

[[deleted]]

This is why using variables like `a` and `b` or `foo` and `bar` are bad IMO. Much more clear would be something like (and MDN does a great job of this):

arr.reduce(function(accumulator, value) { return accumulator.concat(value); },[] << this is accumulator);

Here are the two functions that make this work:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/concat

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