Quoting Mozilla JS Reference on this:

Arrays are list-like objects whose prototype has methods to perform traversal and mutation operations. Neither the length of a JavaScript array nor the types of its elements are fixed.

More on this can be found here

Here's one of the many possible solutions :

var a1 = ["a", "b", "c"];
var a2 = [1, 2, 3];
var a3 = a1.map(function(e,i){ return [e,a2[i]] });

This makes [["a",1],["b",2],["c",3]].

See map

If what you want is an object (an "associative array") like {a: 1, b: 2, c: 3}

then it's a little more interesting :

var a1 = ["a", "b", "c"];
var a2 = [1, 2, 3];
var a3 = a1.reduce(function(r,e,i){ r[e]=a2[i]; return r }, {});

See reduce

This is the case in any language that is not strongly typed. Your array members can be of different primitive and they also can be objects. In most cases you wouldn't want to use this because there is no clear structure to your array. You would rather have something like:

var data = {
    prop: 12,
    otherProp: 24.5,
    stringProp: "hello",
    boolProp: true
};