Why does having [].map with curly brackets change the way it works?

Question

So, basically I have this:

Array.prototype.toString = function() {
    return (\"[\" + this.map(thing => thing = \'\"\' + thing + \'\"\').join(\', \') + \"]\"         


        

Answer 1

This has nothing to do with arrays or the map method. It is entirely about how arrow functions work.

When you give a single statement on the right hand side, then that statement is evaluated and returned inside the function.

foo => bar

is equivalent to:

function (foo) { return bar; }

When you put a block on the right hand side, that block simply because the function body.

foo => { bar }

is equivalent to:

function (foo) { bar; }

In this second version, you have no return statement, so the function returns undefined.

You need to make the return statement explicit if you use a block.

foo => { return bar; }