check if an object is string in Javascript
I was following a tutorial that suggested to check if an object is string and not empty as the following:
var s = "text here";
if ( s && s.charAt && s.charAt(0))
it is said that if s is string then it has a method charAt and then the last component will check if the string is empty or not.
I tried to test it with the other available methods like ( typeof and instanceof ) using some of the SO questions and here and here too !!
so I decided to test it in Js Bin : jsbin code here as follow:
var string1 = "text here";
var string2 = "";
alert("string1 is " + typeof string1);
alert("string2 is " + typeof string2);
//part1- this will succeed and show it is string
if(string1 && string1.charAt){
alert( "part1- string1 is string");
}else{
alert("part1- string1 is not string ");
}
//part2- this will show that it is not string
if(string2 && string2.charAt ){
alert( "part2- string2 is string");
}else{
alert("part2- string2 is not string ");
}
//part3 a - this also fails !!
if(string2 instanceof String){
alert("part3a- string2 is really a string");
}else{
alert("part3a- failed instanceof check !!");
}
//part3 b- this also fails !!
//i tested to write the String with small 's' => string
// but then no alert will excute !!
if(string2 instanceof string){
alert("part3b- string2 is really a string");
}else{
alert("part3b- failed instanceof check !!");
}
Now my questions are :
1- why does the check for string fails when the string is empty using the string2.charAt ???
2- why does the instanceof check failed??
string values are not String objects (which is why the instanceof fails)2.
To cover both cases using "type-checking" it would be typeof x === "string" || x instanceof String typeof x === "string" || x instanceof String ; the first only matches strings and the latter matches Strings.
The tutorial assumes that [only] String objects - or string values which are promoted1 - have a charAt method and so uses "duck-typing". If the method does exist, then it is called. If charAt is used out-of-bounds then an empty string "", which is a false-y value, is returned.
The tutorial code would also accept a string of " ", while s && s.length would not - but it would also "work" on arrays (or jQuery objects, etc). Personally, I trust the caller to provide the allowed values/types and use as little "type-checking" or special-casing as possible.
1 For primitive values of string, number, and boolean there is a corresponding object type of String, Number, and Boolean, respectively. When x.property is used on one of these primitive values the effect is ToObject(x).property - hence the "promotion". This is discussed in ES5: 9.9 - ToObject.
Neither the null or undefined values have corresponding objects (or methods). Functions are already objects but have a historically different, and useful, typeof result.
2 See ES5: 8 - Types for the different types of values. The String Type, eg., represents a string value.
1- Why does the check for string fails when the string is empty using the string2.charAt?
The following expression evaluates to false because the first condition fails:
var string2 = "";
if (string2 && string2.charAt) { console.log("doesn't output"); }
That second line is basically equivalent to:
if (false && true) { console.log("doesn't output"); }
So for example:
if (string2) { console.log("this doesn't output since string2 == false"); }
if (string2.charAt) { console.log('this outputs'); }
2- Why does the instanceof check fail?
This fails because, in javascript, string can be literals or objects. For example:
var myString = new String("asdf");
myString instanceof String; // true
However:
var myLiteralString = "asdf";
myLiteralString instanceof String; // false
You can reliably tell if it's a string by checking both the type and the instanceof :
str instanceof String || typeof str === "string";