Check if a variable is a string in JavaScript

I needed to check if the variable is a string and wanted to do it "quick and dirty". Here is my approach. Since only string have replaceAll method this is enough eficient.

const isString =  (input) => typeof input?.replaceAll === 'function'
// Examples of usage
console.log(isString(true))
// For both of them it will be true
console.log(isString(new String('test')))
console.log(isString('test'))

In old browsers you could reliably test for string objects and primitives with Object.prototype.toString() which would return '[object String]' only on string values. But with the addition of Symbol.toStringTag in ES6, you can no longer rely on Object.prototype.toString() to always tell you the correct object class.

So to avoid potential false positives and negatives I wrote this function which first checks whether Symbol.toStringTag is a property of the tested value. If so, it tests the value with String.prototype.toString() which throws on non-string values. Otherwise it avoids the unnecessary overhead of try/catch with the faster way of testing for strings with Object.prototype.toString().

Works in both modern and old browsers with 100% accuracy:

var isString = function(){
    function isntString(value){// isolate try/catch for better performance in old browsers
        try{
            ''.toString.call(value);// throws on non-string values
        }
        catch(e) {
            return true
        }
    }
    var toStringTag = typeof Symbol=='function' && Symbol.toStringTag;
    return function isString(value) {
        return typeof value=='object' && value? toStringTag && toStringTag in value?
            !isntString(value): {}.toString.call(value)=='[object String]': typeof value=='string'
    }
}();
isString('');// true
isString(new String);// true
isString({[Symbol.toStringTag]:'String'});// false

A code to have only string without any numbers

isNaN("A") = true;
parseInt("A") = NaN;
isNaN(NaN) = true;

Than we can use isNaN(parseInt()) to have only the string

let ignoreNumbers = "ad123a4m";

let ign = ignoreNumbers.split("").map((ele) => isNaN(parseInt(ele)) ? ele : "").join("");

console.log(ign);

Below are two simple functions that should be fairly fast and work correctly for all the different types of string instances/variables. One for a scalar value check and the other for a vector of values to check.

const isString = x => !!x?.toUpperCase;

['', 'a', new String, String('a'), String(1)].map(v => isString(v));
// [true, true, true, true, true]

[null, undefined, 0, 1, true, [], ['a'], {}, {a: 'a'}].map(v => isString(v));
// [false, false, false, false, false, false, false, false, false]

const isStringArray = x => x.map(v => !!v?.toUpperCase).reduce((a, b) => a && b, true);

isStringArray([]);
// true

isStringArray(['', 'a', new String, String('a'), String(1)]);
// true

isStringArray(['', 'a', new String, String('a'), String(1), 1]);
// false

isStringArray([undefined]);
// false

isStringArray([null]);
// false

isStringArray([true]);
// false

isStringArray([false]);
// false

isStringArray([0]);
// false

isStringArray([1]);
// false

isStringArray([[]]);
// false

isStringArray([['a']]);
// false

isStringArray([{}]);
// false

isStringArray([{a: 'a'}]);
// false