Question

function foo() {}
var bar = foo <| function() {};

This is the first time I’ve seen something like this. What does <| mean?

Source: https://github.com/allenwb/ESnext-experiments/blob/master/ST80collections-exp1.js

Answer 1

Now that you have posted the link to the source, you can see in the comments at the top of the file exactly what it does (line 36):

the <| operator — defines the [[Prototype]] of a literal…

For these examples <| used with a function expression sets the
[[Prototype]] of the object created as the value of the function’s
“prototype” property to the value of the “prototype” property of the
the LHS object. This is in addition to setting the [[Prototype]] of
the function object itself. In other words, it builds sets the
[[Prototype]] of both the function and of function.prototype to
potentially different values.

Update: I’ve just remembered this question as I came across the full ECMAScript Harmony proposal for this “literal [[Prototype]] operator”. There is a lot more information in there than in the quote above, so it’s worth a read.