The callback is passed the element, the index, and the array itself.
arr.forEach(function(part, index, theArray) {
theArray[index] = "hello world";
});
edit — as noted in a comment, the .forEach()
function can take a second argument, which will be used as the value of this
in each call to the callback:
arr.forEach(function(part, index) {
this[index] = "hello world";
}, arr); // use arr as this
That second example shows arr
itself being set up as this
in the callback.One might think that the array involved in the .forEach()
call might be the default value of this
, but for whatever reason it's not; this
will be undefined
if that second argument is not provided.
(Note: the above stuff about this
does not apply if the callback is a =>
function, because this
is never bound to anything when such functions are invoked.)
Also it's important to remember that there is a whole family of similar utilities provided on the Array prototype, and many questions pop up on Stackoverflow about one function or another such that the best solution is to simply pick a different tool. You've got:
forEach
for doing a thing with or to every entry in an array;filter
for producing a new array containing only qualifying entries;map
for making a one-to-one new array by transforming an existing array;some
to check whether at least one element in an array fits some description;every
to check whether all entries in an array match a description;find
to look for a value in an arrayand so on. MDN link