`Array.prototype.map` doesn't pass a reference to the *newly-created* array either. The third argument `Array.prototype.map` passes the mapping function is a reference to the *existing* array, not the new one. With `Array.from`, there is no existing array to pass.
Not having the mapping functions (either of them) get a reference to the array being constructed avoids an entire class of errors caused by the mapping function modifying the array as it's being constructed. -- T.J. Crowder On Wed, May 3, 2017 at 11:50 AM, the kojoman <[email protected]> wrote: > Array.map(mapFn) calls the mapFn(v,i,a) passing the currentValue, > currentIndex and the array it is iterating over as arguments. > I was happy to find out about Array.from(itterable, mapFn, thisArg), but > unfortunately, the mapping didn't work as expected. > It seems, Array.from doesn't pass in the newly created array to the mapFn. > Why is that? > > For instance I was expecting Array.from("aabc", (v,i,a) => { if(a[i] === > a[i-1]) a.splice(i,1) }) to evaluate to ["a","b","c"], but a is undefined. > Is there a reason why the mapFn isn't called the same way as an Array.map > function is? > -John > > _______________________________________________ > es-discuss mailing list > [email protected] > https://mail.mozilla.org/listinfo/es-discuss > >
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