js获取表单中的元素的值

// HTML
<input type="text" id="username">

// JavaScript
let username = document.getElementById('username').value;
console.log(username);

// HTML
<input type="text" class="user-input">

// JavaScript
let username = document.querySelector('.user-input').value;
console.log(username);

// HTML
<input type="text" name="username">

// JavaScript
let usernames = document.getElementsByName('username');
for (let i = 0; i < usernames.length; i++) {
    console.log(usernames[i].value);
}

// HTML
<input type="text">

// JavaScript
let inputs = document.getElementsByTagName('input');
for (let i = 0; i < inputs.length; i++) {
    console.log(inputs[i].value);
}

// HTML
<form id="myForm">
    <input type="text" name="username">
</form>

// JavaScript
let form = document.getElementById('myForm');
let username = form.elements['username'].value;
console.log(username);

window.onload = function() {
    let username = document.getElementById('username').value;
    console.log(username);
};

// 对于复选框
let isChecked = document.getElementById('myCheckbox').checked;

// 对于单选按钮
let selectedRadio = document.querySelector('input[name="myRadio"]:checked').value;