ES6 Validate a UK Telephone number

validate-uk-phone.js

export default function ValidateUKPhone(number) {
	
	const errorMessages = [
		"Please enter a UK telephone number without the country code.",
		"UK telephone numbers should contain 10 or 11 digits.",
		"The telephone number should start with a 0.",
		"The telephone number is either invalid or inappropriate."
	];


	let telnum = `${number}`;


	const countryCodeExpression = /^(\+)[\s]*(.*)$/;

	if(countryCodeExpression.test(telnum) == true) {
		return errorMessages[0];
	}

	while (telnum.indexOf(" ") != -1) {
        telnum = telnum.slice(0, telnum.indexOf(" ")) + telnum.slice(telnum.indexOf(" ") + 1)
    }

    while (telnum.indexOf("-") != -1) {
        telnum = telnum.slice(0, telnum.indexOf("-")) + telnum.slice(telnum.indexOf("-") + 1)
    }

   	const lengthExpression = /^[0-9]{10,11}$/;

    if (lengthExpression.test(telnum) != true) {
        return errorMessages[1];
    }

    const startsWithZeroExpression = /^0[0-9]{9,10}$/;
    if (startsWithZeroExpression.test(telnum) != true) {
        return errorMessages[2];
    }

    const formatExpression = new Array();
    formatExpression.push(/^(0113|0114|0115|0116|0117|0118|0121|0131|0141|0151|0161)(4960)[0-9]{3}$/);
    formatExpression.push(/^02079460[0-9]{3}$/);
    formatExpression.push(/^01914980[0-9]{3}$/);
    formatExpression.push(/^02890180[0-9]{3}$/);
    formatExpression.push(/^02920180[0-9]{3}$/);
    formatExpression.push(/^01632960[0-9]{3}$/);
    formatExpression.push(/^07700900[0-9]{3}$/);
    formatExpression.push(/^08081570[0-9]{3}$/);
    formatExpression.push(/^09098790[0-9]{3}$/);
    formatExpression.push(/^03069990[0-9]{3}$/);

    for(let i = 0, l = formatExpression.length; i < l; i++) {
    	if(formatExpression[i].test(telnum)) {
    		return errorMessages[3];
    	}
    }

    const formatExpression2 = (/^(01|02|03|05|070|071|072|073|074|075|07624|077|078|079)[0-9]+$/);

    if (formatExpression2.test(telnum) != true) {
        return errorMessages[3];
    }

    return true;
}