How to convert a list to a list of lists in Javascript?

For normal browsers, to convert a list to a list of lists in Javascript, you probably want Array.prototype.reduce.

let group = (values) => Object.values( // Convert { a: b } to just [b] values.reduce( (p, v) => ( // p = previous value, v = next value (p[v] ||= []), // Assign p[v] a new array if not yet set p[v].push(v), // Add v to this array p), // Return p for next iteration {}) // Initial state object for reduce );
Which will return an Array of arrays with all the same value.
let source = [1, 1, 1, 2, 3, 3]; group(source); // [[1, 1, 1], [2], [3, 3, 3]]

There's are two experimental methods called Array.prototype.group and Array.prototype.groupToMap that would fundamentally output the same results ({ 1: [1,1,1], 2: [2], 3: [3,3,3] }), but isn't available without polyfills except in Firefox Nightly, so you're better off having a handy function like this if you find yourself needing this sort of grouping often.