Modify object property in an array of objects

You can use find and change its property.

let foo = [{ bar: 1, baz: [1,2,3] }, { bar: 2, baz: [4,5,6] }];

let obj = foo.find(f=>f.bar==1);
if(obj)
  obj.baz=[2,3,4];
console.log(foo);

.map with spread (...) operator

var result = foo.map(el => el.bar == 1 ? {...el, baz: [11,22,33]} : el);

Sure, just change it:

With jQuery's $.each:

$.each(foo, function() {
    if (this.bar === 1) {
        this.baz[0] = 11;
        this.baz[1] = 22;
        this.baz[2] = 33;
        // Or: `this.baz = [11, 22, 33];`
    }
});

With ES5's forEach:

foo.forEach(function(obj) {
    if (obj.bar === 1) {
        obj.baz[0] = 11;
        obj.baz[1] = 22;
        obj.baz[2] = 33;
        // Or: `obj.baz = [11, 22, 33];`
    }
});

...or you have other looping options in this other SO answer.