Text Processing with JavaScript

Recipe 1

Determining If a Value Is a String with the typeof Operator

Task

In Javascript, you will often need to validate that a value is a string. For example, in the next recipe, we’ll build a function that searches for multiple words in a string. Before you pass a value to the function, you’d want to check if it’s a string. Any other data type, like a number or Boolean, would cause your function to throw an error.

Throughout this book, you will see functions of this sort repeatedly, so it’s a good place to start.

Solution

Use the typeof operator:

part_1/determining_valid_strings/typeof_ex1.js

function isString(value) {
  return typeof value === "string";
}

isString(123);      // → false
isString("abc");    // → true

The typeof operator ...