Javascript | Set all values of an array

There's no built-in way, you'll have to loop over all of them:

function setAll(a, v) {
    var i, n = a.length;
    for (i = 0; i < n; ++i) {
        a[i] = v;
    }
}

http://jsfiddle.net/alnitak/xG88A/

If you really want, do this:

Array.prototype.setAll = function(v) {
    var i, n = this.length;
    for (i = 0; i < n; ++i) {
        this[i] = v;
    }
};

and then you could actually do cool.setAll(42) (see http://jsfiddle.net/alnitak/ee3hb/).

Some people frown upon extending the prototype of built-in types, though.

EDIT ES5 introduced a way to safely extend both Object.prototype and Array.prototype without breaking for ... in ... enumeration:

Object.defineProperty(Array.prototype, 'setAll', {
    value: function(v) {
        ...
    }
});

EDIT 2 In ES6 draft there's also now Array.prototype.fill, usage cool.fill(42)