Easiest way to fill array with an object in JavaScript without coding a loop?

There is a way: manually. Any programmatic solution will indeed use a loop here, which, if you wish to maintain your sanity -- use it.

You can do this using .fill

const someArray = ['test']; // Any other data you may have
const resultingArray = new Array(100).fill({ foo: 'bar' }).concat(someArray);

Set 100 to whatever number you need, concat any extra data after (or before if needed)

As @isvforall pointed out this isn't a deep copy and is done via reference so changing any of the elements in the array would affect all of the others which reference the same object. Best used if your values are primitives.

I think epascarello's answer is already a winner, and I +1'd it, but here's another approach just for kicks, using your Array.join() trick along with JSON.stringify() and JSON.parse().

Verbose:

var obj = { foo: 'bar' };
var objJSON = JSON.stringify(obj);
var list = new Array(501).join(objJSON + ',')
var trimmedList = list.substr(0, list.length - 1); // getting rid of that last comma
var arrayJSON = '[' + trimmedList + ']';
var array = JSON.parse(arrayJSON);
console.log(array);

Compact:

var list = new Array(501).join(JSON.stringify({ foo: 'bar' }) + ',')
var array = JSON.parse('[' + list.substr(0, list.length - 1) + ']');
console.log(array);

If you really don't want a loop you can use recursion.

function objFillArray(obj, count) {
    let ret = [];
    return notALoop();
    function notALoop() {
        ret.push(obj);
        if( --count ) return notALoop();
        return ret;
    }
}

this will keep calling notALoop count number of times adding the object to the returned array.

Using recursion and concat function

var obj = { foo: 'bar' };

var result = (function fill(a, len) {
    return a.length == len ? a : fill(a.concat(obj), len);
})([], 10);

var obj = { foo: 'bar' };

var result = (function fill(a, len) {
    return a.length == len ? a : fill(a.concat(obj), len);
})([], 10);

document.write(JSON.stringify(result));

The callback function in Array.from can be used to ensure each element of the array is a different reference.

const arr = Array.from({length: 5}, _=>({foo: 'bar'}));
console.log(arr);

If all elements are meant to be the same reference, directly use Array#fill.

const arr = Array(5).fill({foo: 'bar'});
console.log(arr);

Use .push() function to assign value to an array.

    yourarray.push(yourobject);
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    fiveHundredChars.push(obj);