function verifyNumber(value) {
let reg = /^[+]{0,1}(\d+)$|^[+]{0,1}(\d+\.\d+)$/;
let lastChar = value.substr(value.length - 1);
return reg.test(lastChar) || value === "";
}
verifyNumber("-1") //true
verifyNumber("+1") //true
verifyNumber("1") //true
verifyNumber("1a") //false