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

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);