The method given in the ECMAScript standard to find the class of Object is to use the toString method from Object.prototype.

if( someVar ) === '[object Array]' ) {
    alert( 'Array!' );

Or you could use typeof to test if it is a String:

if( typeof someVar === 'string' ) {
    someVar = [ someVar ];

Or if you're not concerned about performance, you could just do a concat to a new empty Array.

someVar = [].concat( someVar );

There's also the constructor which you can query directly:

if ( == "Array") {
    // do something

Check out a thorough treatment from @T.J. Crowder's blog, as posted in his comment below.

Check out this benchmark to get an idea which method performs better:

From @Bharath convert string to array using Es6 for the question asked:

const convertStringToArray = (object) => {
   return (typeof object === 'string') ? Array(object) : object 


let m = 'bla'
let n = ['bla','Meow']
let y = convertStringToArray(m)
let z = convertStringToArray(n)
console.log('check y: '+JSON.stringify(y)) . // check y: ['bla']
console.log('check y: '+JSON.stringify(z)) . // check y: ['bla','Meow']