How to do equivalent of LINQ SelectMany() just in javascript

For those a while later, understanding javascript but still want a simple Typed SelectMany method in Typescript:

function selectMany<TIn, TOut>(input: TIn[], selectListFn: (t: TIn) => TOut[]): TOut[] {
  return input.reduce((out, inx) => {
    out.push(...selectListFn(inx));
    return out;
  }, new Array<TOut>());
}

for a simple select you can use the reduce function of Array.
Lets say you have an array of arrays of numbers:

var arr = [[1,2],[3, 4]];
arr.reduce(function(a, b){ return a.concat(b); }, []);
=>  [1,2,3,4]

var arr = [{ name: "name1", phoneNumbers : [5551111, 5552222]},{ name: "name2",phoneNumbers : [5553333] }];
arr.map(function(p){ return p.phoneNumbers; })
   .reduce(function(a, b){ return a.concat(b); }, [])
=>  [5551111, 5552222, 5553333]

Edit:
since es6 flatMap has been added to the Array prototype. SelectMany is synonym to flatMap.
The method first maps each element using a mapping function, then flattens the result into a new array. Its simplified signature in TypeScript is:

function flatMap<A, B>(f: (value: A) => B[]): B[]

In order to achieve the task we just need to flatMap each element to phoneNumbers

arr.flatMap(a => a.phoneNumbers);

As a simpler option Array.prototype.flatMap() or Array.prototype.flat()

const data = [
{id: 1, name: 'Dummy Data1', details: [{id: 1, name: 'Dummy Data1 Details'}, {id: 1, name: 'Dummy Data1 Details2'}]},
{id: 1, name: 'Dummy Data2', details: [{id: 2, name: 'Dummy Data2 Details'}, {id: 1, name: 'Dummy Data2 Details2'}]},
{id: 1, name: 'Dummy Data3', details: [{id: 3, name: 'Dummy Data3 Details'}, {id: 1, name: 'Dummy Data3 Details2'}]},
]

const result = data.flatMap(a => a.details); // or data.map(a => a.details).flat(1);
console.log(result)