Test for array of string type in TypeScript

Question

You cannot test for string[] in the general case but you can test for Array quite easily the same as in JavaScript https://stackoverflow.com/a/767492/390330 (I prefer Array.isArray(value)).

If you specifically want for string array you can do something like:

if (Array.isArray(value)) {
   var somethingIsNotString = false;
   value.forEach(function(item){
      if(typeof item !== 'string'){
         somethingIsNotString = true;
      }
   })
   if(!somethingIsNotString && value.length > 0){
      console.log('string[]!');
   }
}

Another option is Array.isArray()

if(! Array.isArray(classNames) ){
    classNames = [classNames]
}

Here is the most concise solution so far:

function isArrayOfStrings(value: any): boolean {
   return Array.isArray(value) && value.every(item => typeof item === "string");
}

Note that value.every will return true for an empty array. If you need to return false for an empty array, you should add value.length to the condition clause:

function isNonEmptyArrayOfStrings(value: any): boolean {
    return Array.isArray(value) && value.length && value.every(item => typeof item === "string");
}

There is no any run-time type information in TypeScript (and there won't be, see TypeScript Design Goals > Non goals, 5), so there is no way to get the type of an empty array. For a non-empty array all you can do is to check the type of its items, one by one.

I know this has been answered, but TypeScript introduced type guards: https://www.typescriptlang.org/docs/handbook/advanced-types.html#typeof-type-guards

If you have a type like: Object[] | string[] and what to do something conditionally based on what type it is - you can use this type guarding:

function isStringArray(value: any): value is string[] {
  if (value instanceof Array) {
    value.forEach(function(item) { // maybe only check first value?
      if (typeof item !== 'string') {
        return false
      }
    })
    return true
  }
  return false
}

function join<T>(value: string[] | T[]) {
  if (isStringArray(value)) {
    return value.join(',') // value is string[] here
  } else {
    return value.map((x) => x.toString()).join(',') // value is T[] here
  }
}

There is an issue with an empty array being typed as string[], but that might be okay

Try this:

if (value instanceof Array) {
alert('value is Array!');
} else {
alert('Not an array');
}

You can have do it easily using Array.prototype.some() as below.

const isStringArray = (test: any[]): boolean => {
 return Array.isArray(test) && !test.some((value) => typeof value !== 'string')
}
const myArray = ["A", "B", "C"]
console.log(isStringArray(myArray)) // will be log true if string array

I believe this approach is better that others. That is why I am posting this answer.

Update on Sebastian Vittersø's comment

Here you can use Array.prototype.every() as well.

const isStringArray = (test: any[]): boolean => {
 return Array.isArray(test) && test.every((value) => typeof value === 'string')
}