[javascript] How can I check if a var is a string in JavaScript?

How can I check if a var is a string in JavaScript?

I've tried this and it doesn't work...

var a_string = "Hello, I'm a string.";

if (a_string typeof 'string') {
    // this is a string
}

The typeof operator isn't an infix (so the LHS of your example doesn't make sense).

You need to use it like so...

if (typeof a_string == 'string') {
    // This is a string.
}

Remember, typeof is an operator, not a function. Despite this, you will see typeof(var) being used a lot in the wild. This makes as much sense as var a = 4 + (1).

Also, you may as well use == (equality comparison operator) since both operands are Strings (typeof always returns a String), JavaScript is defined to perform the same steps had I used === (strict comparison operator).

As Box9 mentions, this won't detect a instantiated String object.

You can detect for that with....

var isString = str instanceof String;

jsFiddle.

...or...

var isString = str.constructor == String;

jsFiddle.

But this won't work in a multi window environment (think iframes).

You can get around this with...

var isString = Object.prototype.toString.call(str) == '[object String]';

jsFiddle.

But again, (as Box9 mentions), you are better off just using the literal String format, e.g. var str = 'I am a string';.

Further Reading.

check for null or undefined in all cases a_string

if (a_string && typeof a_string === 'string') {
    // this is a string and it is not null or undefined.
}

Now days I believe it's preferred to use a function form of typeof() so...

if(filename === undefined || typeof(filename) !== "string" || filename === "") {
   console.log("no filename aborted.");
   return;
}

Following expression returns true:

'qwe'.constructor === String

Following expression returns true:

typeof 'qwe' === 'string'

Following expression returns false (sic!):

typeof new String('qwe') === 'string'

Following expression returns true:

typeof new String('qwe').valueOf() === 'string'

Best and right way (imho):

if (someVariable.constructor === String) {
   ...
}

Combining the previous answers provides these solutions:

if (typeof str == 'string' || str instanceof String)

or

Object.prototype.toString.call(str) == '[object String]'

My personal approach, which seems to work for all cases, is testing for the presence of members that will all only be present for strings.

function isString(x) {
    return (typeof x == 'string' || typeof x == 'object' && x.toUpperCase && x.substr && x.charAt && x.trim && x.replace ? true : false);
}

See: http://jsfiddle.net/x75uy0o6/

I'd like to know if this method has flaws, but it has served me well for years.