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Check if a variable is a string in JavaScript


Question

How can I determine whether a variable is a string or something else in JavaScript?

2017/11/07
1
1820
11/7/2017 3:55:53 PM

Accepted Answer

You can use typeof operator:

var booleanValue = true; 
var numericalValue = 354;
var stringValue = "This is a String";
var stringObject = new String( "This is a String Object" );
alert(typeof booleanValue) // displays "boolean"
alert(typeof numericalValue) // displays "number"
alert(typeof stringValue) // displays "string"
alert(typeof stringObject) // displays "object"

Example from this webpage. (Example was slightly modified though).

This won't work as expected in the case of strings created with new String(), but this is seldom used and recommended against[1][2]. See the other answers for how to handle these, if you so desire.


  1. The Google JavaScript Style Guide says to never use primitive object wrappers.
  2. Douglas Crockford recommended that primitive object wrappers be deprecated.
2018/11/06
1781
11/6/2018 8:14:02 AM


Since 580+ people have voted for an incorrect answer, and 800+ have voted for a working but shotgun-style answer, I thought it might be worth redoing my answer in a simpler form that everybody can understand.

function isString(x) {
  return Object.prototype.toString.call(x) === "[object String]"
}

Or, inline (I have an UltiSnip setup for this):

Object.prototype.toString.call(myVar) === "[object String]"

FYI, Pablo Santa Cruz's answer is wrong, because typeof new String("string") is object

DRAX's answer is accurate and functional, and should be the correct answer (since Pablo Santa Cruz is most definitely incorrect, and I won't argue against the popular vote.)

However, this answer is also definitely correct, and actually the best answer (except, perhaps, for the suggestion of using lodash/underscore). disclaimer: I contributed to the lodash 4 codebase.

My original answer (which obviously flew right over a lot of heads) follows:

I transcoded this from underscore.js:

['Arguments', 'Function', 'String', 'Number', 'Date', 'RegExp'].forEach( 
    function(name) { 
        window['is' + name] = function(obj) {
              return toString.call(obj) == '[object ' + name + ']';
    }; 
});

That will define isString, isNumber, etc.


In Node.js, this can be implemented as a module:

module.exports = [
  'Arguments',
  'Function', 
  'String', 
  'Number', 
  'Date', 
  'RegExp'
].reduce( (obj, name) => {
  obj[ 'is' + name ] = x => toString.call(x) == '[object ' + name + ']';
  return obj;
}, {});

[edit]: Object.prototype.toString.call(x) works to delineate between functions and async functions as well:

const fn1 = () => new Promise((resolve, reject) => setTimeout(() => resolve({}), 1000))
const fn2 = async () => ({})

console.log('fn1', Object.prototype.toString.call(fn1))
console.log('fn2', Object.prototype.toString.call(fn2))

2020/05/26

I recommend using the built-in functions from jQuery or lodash/Underscore. They're simpler to use and easier to read.

Either function will handle the case DRAX mentioned... that is, they both check if (A) the variable is a string literal or (B) it's an instance of the String object. In either case, these functions correctly identify the value as being a string.

lodash / Underscore.js

if(_.isString(myVar))
   //it's a string
else
   //it's something else

jQuery

if($.type(myVar) === "string")
   //it's a string
else
   //it's something else

See lodash Documentation for _.isString() for more details.

See jQuery Documentation for $.type() for more details.

2015/09/22

function isString (obj) {
  return (Object.prototype.toString.call(obj) === '[object String]');
}

I saw that here:

http://perfectionkills.com/instanceof-considered-harmful-or-how-to-write-a-robust-isarray/

2016/07/08

Best way:

var s = 'String';
var a = [1,2,3];
var o = {key: 'val'};

(s.constructor === String) && console.log('its a string');
(a.constructor === Array) && console.log('its an array');
(o.constructor === Object) && console.log('its an object');
(o.constructor === Number || s.constructor === Boolean) && console.log('this won\'t run');

Each of these has been constructed by its appropriate class function, like "new Object()" etc.

Also, Duck-Typing: "If it looks like a duck, walks like a duck, and smells like a duck - it must be an Array" Meaning, check its properties.

Hope this helps.

Edit; 12/05/2016

Remember, you can always use combinations of approaches too. Here's an example of using an inline map of actions with typeof:

var type = { 'number': Math.sqrt.bind(Math), ... }[ typeof datum ];

Here's a more 'real world' example of using inline-maps:

function is(datum) {
    var isnt = !{ null: true, undefined: true, '': true, false: false, 0: false }[ datum ];
    return !isnt;
}
console.log( is(0), is(false), is(undefined), ... );  // >> true true false

This function would use [ custom ] "type-casting" -- rather, "type-/-value-mapping" -- to figure out if a variable actually "exists". Now you can split that nasty hair between null & 0!

Many times you don't even care about its type. Another way to circumvent typing is combining Duck-Type sets:

this.id = "998";  // use a number or a string-equivalent
function get(id) {
    if (!id || !id.toString) return;
    if (id.toString() === this.id.toString()) http( id || +this.id );
    // if (+id === +this.id) ...;
}

Both Number.prototype and String.prototype have a .toString() method. You just made sure that the string-equivalent of the number was the same, and then you made sure that you passed it into the http function as a Number. In other words, we didn't even care what its type was.

Hope that gives you more to work with :)

2020/06/20

I can't honestly see why one would not simply use typeof in this case:

if (typeof str === 'string') {
  return 42;
}

Yes it will fail against object-wrapped strings (e.g. new String('foo')) but these are widely regarded as a bad practice and most modern development tools are likely to discourage their use. (If you see one, just fix it!)

The Object.prototype.toString trick is something that all front-end developers have been found guilty of doing one day in their careers but don't let it fool you by its polish of clever: it will break as soon as something monkey-patch the Object prototype:

const isString = thing => Object.prototype.toString.call(thing) === '[object String]';

console.log(isString('foo'));

Object.prototype.toString = () => 42;

console.log(isString('foo'));

2019/01/18

Source: https://stackoverflow.com/questions/4059147
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