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A curious usage of commas in JavaScript

Updated Jul 11 2020

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I recently discovered a curious yet possibly useful thing in JavaScript, related to the comma operator.

I mostly use commas to separate properties in an object, or array items.

However I never gave much attention the usage of it inside an expression.

Take this:

('a', 'b')

Both expressions (in this case strings) are evaluated, and this returns the last element, the expression after the last comma. In this example it returns 'b'.

You can assign the value to a variable, like this:

const letter = ('a', 'b')
letter === 'b' //true

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