Bdtjtk

How can I test if a variable is an array of string in TypeScript? Something like this:

function f(): string {

    var a: string = ["A", "B", "C"];



    if (typeof a === "string")    {

        return "Yes"

    }

    else {

        // returns no as it's 'object'

        return "No"

    }

};

TypeScript.io here: http://typescript.io/k0ZiJzso0Qg/2

Edit: I've updated the text to ask for a test for string. This was only in the code example previously.

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

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

if (value instanceof Array) {

   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 isArrayOfString(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 way to check the type of an empty array since there is no type information at runtime.

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');

}

there is a little problem here because the

if (typeof item !== 'string') {

    return false

}

will not stop the foreach.
So the function will return true even if the array does contain none string values.

This seems to wok for me:

function isStringArray(value: any): value is number {

  if (Object.prototype.toString.call(value) === '[object Array]') {

     if (value.length < 1) {

       return false;

     } else {

       return value.every((d: any) => typeof d === 'string');

     }

  }

  return false;

}

Greetings, Hans