Bootstrap mobile menu icon change to x close

Question

EDIT

@zim answer uses 2020 CSS to easily solve the issue and better applies to Bootstrap 4.

The original question and selected answer are still

Answer 1

You don't need Javascript, CSS will do the Job

.navbar-toggle {
    .icon-bar {
        transition: 300ms ease-in-out;
        background-color: #fff;
        position: relative;
        width: 24px;
        height: 3px;
    }
    .icon-bar:last-child {
        -webkit-transform: rotate(-45deg);
        -ms-transform: rotate(-45deg);
        -o-transform: rotate(-45deg);
        transform: rotate(-45deg);
        top: -7px;
    }
    .icon-bar:nth-child(2) {
        -webkit-transform: rotate(45deg);
        -ms-transform: rotate(45deg);
        -o-transform: rotate(45deg);
        transform: rotate(45deg);
        top: 0px;
    }
    .icon-bar:nth-child(3) {
        opacity: 0;
    }
    &.collapsed {
        .icon-bar {
            -webkit-transform: rotate(0deg);
            -ms-transform: rotate(0deg);
            -o-transform: rotate(0deg);
            transform: rotate(0deg);        
            top: 0;
            opacity: 1;
        }
    }
}