JSFiddle - React, Tailwind, and code Playground

HTML

<form class="form_box prefix_clear" method="get" action="#login">
    <div class="f-field">
        <label for="f-name">Username:</label><br/>
        <input id="f-name" class="submit_text" type="text" name="username"/>
        <p class="f-error case1">Please enter your username</p>
        <p class="f-error case2">Your username has to be at least 4 characters</p>
    </div>
    <div class="f-field">
        <label for="f-email">Email:</label><br/>
        <input id="f-email" class="submit_text" type="text" name="email"/>
        <p class="f-error case1">Please enter your email</p>
        <p class="f-error case2">Your email is not a valid format</p>
    </div>
    <div  class="f-field">
        <label for="f-pwd">Password:</label><br/>
        <input id="f-pwd" class="submit_text" type="password" name="password"/>
        <p class="f-error case1">Please enter your password</p>
        <p class="f-error case2">Your password has to be at least 6 letters (A-Z, a-z, 0-9)</p>
    </div>
    <div class="prefix_clear">
        <input id="submit_btn" class="f-btn" type="submit" name="submit" value="Create Account"/>
    </div>
</form>

JavaScript

$('#submit_btn').attr('disabled', 'disabled');

var nameFlag = false;;
var emailFlag = false;
var pwdFlag = false;

$('#f-name').focusout(function() {
    if($(this).val() == '') {
        nameFlag = false;
    } else if($(this).val().length < 4) {
        nameFlag = false;
    } else {
        nameFlag = true;
        //alert(nameFlag);
    }
     checkform();
});


$('#f-email').focusout(function() {
    if($(this).val() == '') {
        emailFlag = false;
    } else if($(this).val().length < 4) {
        emailFlag = false;
    } else {
        emailFlag = true;
        //alert(emailFlag);
    }
     checkform();
});


$('#f-pwd').focusout(function() {
    if($(this).val() == '') {
        pwdFlag = false;
    } else if($(this).val().length < 6) {
        pwdFlag = false;
    } else {
        pwdFlag = true;
        //alert(pwdFlag);
    }
     checkform();
});
function checkform(){
//alert(nameFlag + " " + emailFlag + " " + pwdFlag);
if(nameFlag && emailFlag && pwdFlag) {
     $('#submit_btn').removeAttr('disabled');
} else {
    $('#submit_btn').attr('disabled', 'disabled');
}
}