Create slug from string in Javascript

slugify.js

function string_to_slug (str) {
    str = str.replace(/^\s+|\s+$/g, ''); // trim
    str = str.toLowerCase();
  
    // remove accents, swap ñ for n, etc
    var from = "àáäâèéëêìíïîòóöôùúüûñç·/_,:;";
    var to   = "aaaaeeeeiiiioooouuuunc------";
    for (var i=0, l=from.length ; i<l ; i++) {
        str = str.replace(new RegExp(from.charAt(i), 'g'), to.charAt(i));
    }

    str = str.replace(/[^a-z0-9 -]/g, '') // remove invalid chars
        .replace(/\s+/g, '-') // collapse whitespace and replace by -
        .replace(/-+/g, '-'); // collapse dashes

    return str;
}

@auvansang How should the character "đ" be replaced for you—by "d" or another letter?

Yes d is the correct letter

If "đ" was replaced by "d" it is theoretically possible (albeit not likely) to generate a duplicate slug.
For example if I hat a slug "mad" and generating a new slug the input was "mađ" then I'd have a collision: two "mad" slugs.
For some applications this might be totally irrelevant, because the URLs have a tipically numeric id before the slug, preventing address duplication.
So, the decision on how to replace "đ" depends on the context.