Question

Why can’t I do the following?

var str = "A string with * and ^ and even $";
var indeces = ["*", "^", "$"].map(str.indexOf); // doesnt work

I’d rather not pass in another function to do the exact same thing as the method itself…

[...].map(function(token) { return str.indexOf(token); }); // works

Is there an elegant way to do this?

Answer 1

The difference between

function(token) { return str.indexOf(token); }

and

str.indexOf

is that you wouldn’t have context problem in the first case.

In the second case, this would no be str but window. str.indexOf is the value of the property named indexOf of the string. It’s a function but there is no way from the value to know from which object you took it. So if you pass str.indexOf, you’re just passing the function, not at all str.

["*", "^", "$"].map(str.indexOf);

would be equivalent to

["*", "^", "$"].map(function(token) { return window.indexOf(token); });

if there was a indexOf function on window.

Just to try to be clearer, but don’t use it : a function could be bound to an object. If you do

var f = str.indexOf.bind(str); // makes a copy of indexOf, bound to str
["*", "^", "$"].map(f);

then this is str when f is called.