Javascript Form Serialize with pure JS, modified from "https://code.google.com/archive/p/form-serialize/"

serialize.js

function serialize(form, hashmode) {
	if (!form || form.nodeName !== "FORM") {
		return
	}
	var i, j, q = [];
	var hashmap = {};
	for (i = form.elements.length - 1; i >= 0; i = i - 1) {
		if (form.elements[i].name === "") {
			continue
		}
		switch (form.elements[i].nodeName) {
			case 'INPUT':
				switch (form.elements[i].type) {
					case 'text':
					case 'hidden':
					case 'password':
					case 'button':
					case 'reset':
					case 'submit':
						q.push(form.elements[i].name + "=" + encodeURIComponent(form.elements[i].value));
						hashmap[form.elements[i].name] = form.elements[i].value;
						break;
					case 'checkbox':
						if (form.elements[i].checked) {
							q.push(form.elements[i].name + "[]=" + encodeURIComponent(form.elements[i].value));
							// hashmap[form.elements[i].name] = form.elements[i].value
							if(hashmap[form.elements[i].name]) {
								hashmap[form.elements[i].name].push(form.elements[i].value)
							} else {
								hashmap[form.elements[i].name] = [form.elements[i].value]
							}
						}
						break;
					case 'radio':
						if (form.elements[i].checked) {
							q.push(form.elements[i].name + "=" + encodeURIComponent(form.elements[i].value));
							hashmap[form.elements[i].name] = form.elements[i].value
						}
						break;
					case 'file':
						break
				}
				break;
			case 'TEXTAREA':
				q.push(form.elements[i].name + "=" + encodeURIComponent(form.elements[i].value));
				hashmap[form.elements[i].name] = form.elements[i].value;
				break;
			case 'SELECT':
				switch (form.elements[i].type) {
					case 'select-one':
						q.push(form.elements[i].name + "=" + encodeURIComponent(form.elements[i].value));
						hashmap[form.elements[i].name] = form.elements[i].value;
						break;
					case 'select-multiple':
						for (j = form.elements[i].options.length - 1; j >= 0; j = j - 1) {
							if (form.elements[i].options[j].selected) {
								q.push(form.elements[i].name + "=" + encodeURIComponent(form.elements[i].options[j].value));
								hashmap[form.elements[i].name] = form.elements[i].value
							}
						}
						break
				}
				break;
			case 'BUTTON':
				switch (form.elements[i].type) {
					case 'reset':
					case 'submit':
					case 'button':
						q.push(form.elements[i].name + "=" + encodeURIComponent(form.elements[i].value));
						hashmap[form.elements[i].name] = form.elements[i].value;
						break
				}
				break
		}
	}
	if (hashmode) return hashmap;
	return q.join("&")
}

serialize.min.js

function serialize(e,s){if(e&&"FORM"===e.nodeName){var n,t,l=[],m={};for(n=e.elements.length-1;n>=0;n-=1)if(""!==e.elements[n].name)switch(e.elements[n].nodeName){case"INPUT":switch(e.elements[n].type){case"text":case"hidden":case"password":case"button":case"reset":case"submit":l.push(e.elements[n].name+"="+encodeURIComponent(e.elements[n].value)),m[e.elements[n].name]=e.elements[n].value;break;case"checkbox":e.elements[n].checked&&(l.push(e.elements[n].name+"[]="+encodeURIComponent(e.elements[n].value)),m[e.elements[n].name]?m[e.elements[n].name].push(e.elements[n].value):m[e.elements[n].name]=[e.elements[n].value]);break;case"radio":e.elements[n].checked&&(l.push(e.elements[n].name+"="+encodeURIComponent(e.elements[n].value)),m[e.elements[n].name]=e.elements[n].value)}break;case"TEXTAREA":l.push(e.elements[n].name+"="+encodeURIComponent(e.elements[n].value)),m[e.elements[n].name]=e.elements[n].value;break;case"SELECT":switch(e.elements[n].type){case"select-one":l.push(e.elements[n].name+"="+encodeURIComponent(e.elements[n].value)),m[e.elements[n].name]=e.elements[n].value;break;case"select-multiple":for(t=e.elements[n].options.length-1;t>=0;t-=1)e.elements[n].options[t].selected&&(l.push(e.elements[n].name+"="+encodeURIComponent(e.elements[n].options[t].value)),m[e.elements[n].name]=e.elements[n].value)}break;case"BUTTON":switch(e.elements[n].type){case"reset":case"submit":case"button":l.push(e.elements[n].name+"="+encodeURIComponent(e.elements[n].value)),m[e.elements[n].name]=e.elements[n].value}}return s?m:l.join("&")}}

2023.10.30 adapt same name multi-checkbox.