Is there some way I can “join” the contents of two javascript arrays much like I would do a join in SQL

I think what you want is an inner join, which is simple enough to implement in JavaScript:

const innerJoin = (xs, ys, sel) =>
    xs.reduce((zs, x) =>
    ys.reduce((zs, y) =>        // cartesian product - all combinations
    zs.concat(sel(x, y) || []), // filter out the rows and columns you want
    zs), []);

For the purpose of demonstration we’ll use the following data set (thank you @AshokDamani):

const userProfiles = [
    {id: 1, name: "Ashok"},
    {id: 2, name: "Amit"},
    {id: 3, name: "Rajeev"},
];

const questions = [
    {id: 1, text: "text1", createdBy: 2},
    {id: 2, text: "text2", createdBy: 2},
    {id: 3, text: "text3", createdBy: 1},
    {id: 4, text: "text4", createdBy: 2},
    {id: 5, text: "text5", createdBy: 3},
    {id: 6, text: "text6", createdBy: 3},
];

This is how you would use it:

const result = innerJoin(userProfiles, questions,
    ({id: uid, name}, {id, text, createdBy}) =>
        createdBy === uid && {id, text, name});

In SQL terms this would be similar to:

SELECT questions.id, questions.text, userProfiles.name
FROM userProfiles INNER JOIN questions
ON questions.createdBy = userProfiles.id;

Putting it all together:

const innerJoin = (xs, ys, sel) =>
    xs.reduce((zs, x) =>
    ys.reduce((zs, y) =>        // cartesian product - all combinations
    zs.concat(sel(x, y) || []), // filter out the rows and columns you want
    zs), []);

const userProfiles = [
    {id: 1, name: "Ashok"},
    {id: 2, name: "Amit"},
    {id: 3, name: "Rajeev"},
];

const questions = [
    {id: 1, text: "text1", createdBy: 2},
    {id: 2, text: "text2", createdBy: 2},
    {id: 3, text: "text3", createdBy: 1},
    {id: 4, text: "text4", createdBy: 2},
    {id: 5, text: "text5", createdBy: 3},
    {id: 6, text: "text6", createdBy: 3},
];

const result = innerJoin(userProfiles, questions,
    ({id: uid, name}, {id, text, createdBy}) =>
        createdBy === uid && {id, text, name});

console.log("Open your browser console to see the output.");

console.table(result);

Edit: However this is not the best solution. Since the above solution loops through the Cartesian product it takes O(m × n) time to run. With a little bit of modification we can make it run in O(m + n) time – @pebbl found it first:

const equijoin = (xs, ys, primary, foreign, sel) => {
    const ix = xs.reduce((ix, row) => // loop through m items
        ix.set(row[primary], row),    // populate index for primary table
    new Map);                         // create an index for primary table

    return ys.map(row =>              // loop through n items
        sel(ix.get(row[foreign]),     // get corresponding row from primary
        row));                        // select only the columns you need
};

Now you could use it as follows:

const result = equijoin(userProfiles, questions, "id", "createdBy",
    ({name}, {id, text}) => ({id, text, name}));

Putting it all together:

const equijoin = (xs, ys, primary, foreign, sel) => {
    const ix = xs.reduce((ix, row) => ix.set(row[primary], row), new Map);
    return ys.map(row => sel(ix.get(row[foreign]), row));
};

const userProfiles = [
    {id: 1, name: "Ashok"},
    {id: 2, name: "Amit"},
    {id: 3, name: "Rajeev"},
];

const questions = [
    {id: 1, text: "text1", createdBy: 2},
    {id: 2, text: "text2", createdBy: 2},
    {id: 3, text: "text3", createdBy: 1},
    {id: 4, text: "text4", createdBy: 2},
    {id: 5, text: "text5", createdBy: 3},
    {id: 6, text: "text6", createdBy: 3},
];

const result = equijoin(userProfiles, questions, "id", "createdBy",
    ({name}, {id, text}) => ({id, text, name}));

console.log("Open your browser console to see the output.");

console.table(result);