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isString

Determines whether a value is a string type with enhanced type safety. Provides reliable string type detection using native typeof check, distinguishing primitive string values from string-like objects or other data types that might contain text.

Signature​

const isString: (value?: unknown) => value is string

Parameters​

NameTypeDescription
value-Value to test for string type

Returns​

Type-safe boolean indicating whether the value is a string

Examples​

Basic string detection​

import { isString } from '@winglet/common-utils';

// True cases - string type
console.log(isString('hello')); // true
console.log(isString('')); // true (empty string)
console.log(isString('123')); // true (numeric string)
console.log(isString('true')); // true (boolean string)
console.log(isString(`template string`)); // true (template literal)
console.log(isString(String(42))); // true (String constructor result)
console.log(isString('null')); // true (string containing 'null')

// False cases - not string type
console.log(isString(123)); // false (number)
console.log(isString(true)); // false (boolean)
console.log(isString(null)); // false
console.log(isString(undefined)); // false
console.log(isString(new String('hello'))); // false (String object, not primitive)
console.log(isString({})); // false (object)
console.log(isString([])); // false (array)

Form validation with string checking​

interface FormData {
  name: unknown;
  email: unknown;
  message: unknown;
}

function validateStringFields(formData: FormData) {
  const errors: string[] = [];

  if (!isString(formData.name)) {
    errors.push('Name must be a string');
  } else if (formData.name.trim().length === 0) {
    errors.push('Name cannot be empty');
  }

  if (!isString(formData.email)) {
    errors.push('Email must be a string');
  } else if (!formData.email.includes('@')) {
    errors.push('Email must contain @ symbol');
  }

  if (!isString(formData.message)) {
    errors.push('Message must be a string');
  } else if (formData.message.length < 10) {
    errors.push('Message must be at least 10 characters');
  }

  return errors;
}

Text processing with type safety​

function processText(input: unknown): string {
  if (!isString(input)) {
    throw new Error('Input must be a string');
  }

  // TypeScript knows input is string
  return input
    .trim()
    .toLowerCase()
    .replace(/\\s+/g, ' ')
    .split(' ')
    .map(word => word.charAt(0).toUpperCase() + word.slice(1))
    .join(' ');
}

// Usage
console.log(processText('  hello   world  ')); // 'Hello World'
// processText(123) // throws Error: Input must be a string

API parameter validation​

interface SearchParams {
  query?: unknown;
  category?: unknown;
  sortBy?: unknown;
}

function validateSearchParams(params: SearchParams) {
  const validated = {
    query: '',
    category: 'all',
    sortBy: 'relevance'
  };

  if (params.query !== undefined) {
    if (!isString(params.query)) {
      throw new Error('Query must be a string');
    }
    if (params.query.trim().length === 0) {
      throw new Error('Query cannot be empty');
    }
    validated.query = params.query.trim();
  }

  if (params.category !== undefined) {
    if (!isString(params.category)) {
      throw new Error('Category must be a string');
    }
    validated.category = params.category;
  }

  if (params.sortBy !== undefined) {
    if (!isString(params.sortBy)) {
      throw new Error('SortBy must be a string');
    }
    const allowedSorts = ['relevance', 'date', 'popularity'];
    if (!allowedSorts.includes(params.sortBy)) {
      throw new Error(`SortBy must be one of: ${allowedSorts.join(', ')}`);
    }
    validated.sortBy = params.sortBy;
  }

  return validated;
}

Array processing with string filtering​

function extractStrings(values: unknown[]): string[] {
  return values.filter(isString);
}

function processStringArray(values: unknown[]): {
  strings: string[];
  nonEmpty: string[];
  wordCount: number;
  totalLength: number;
} {
  const strings = extractStrings(values);
  const nonEmpty = strings.filter(s => s.trim().length > 0);

  const wordCount = nonEmpty.reduce((count, str) => {
    return count + str.trim().split(/\\s+/).length;
  }, 0);

  const totalLength = strings.reduce((sum, str) => sum + str.length, 0);

  return { strings, nonEmpty, wordCount, totalLength };
}

// Usage
const mixed = ['hello', 42, 'world', null, '', 'test', true];
const result = processStringArray(mixed);
console.log(result);
// {
//   strings: ['hello', 'world', '', 'test'],
//   nonEmpty: ['hello', 'world', 'test'],
//   wordCount: 3,
//   totalLength: 14
// }

Configuration validation​

interface Config {
  apiUrl?: unknown;
  secretKey?: unknown;
  environment?: unknown;
}

function validateConfig(config: Config) {
  const errors: string[] = [];

  if (config.apiUrl !== undefined) {
    if (!isString(config.apiUrl)) {
      errors.push('API URL must be a string');
    } else if (!/^https?:\\/\\//i.test(config.apiUrl)) {
      errors.push('API URL must be a valid HTTP/HTTPS URL');
    }
  }

  if (config.secretKey !== undefined) {
    if (!isString(config.secretKey)) {
      errors.push('Secret key must be a string');
    } else if (config.secretKey.length < 32) {
      errors.push('Secret key must be at least 32 characters');
    }
  }

  if (config.environment !== undefined) {
    if (!isString(config.environment)) {
      errors.push('Environment must be a string');
    } else {
      const validEnvs = ['development', 'staging', 'production'];
      if (!validEnvs.includes(config.environment)) {
        errors.push(`Environment must be one of: ${validEnvs.join(', ')}`);
      }
    }
  }

  return errors;
}

Template processing​

function processTemplate(template: unknown, variables: Record<string, string>) {
  if (!isString(template)) {
    throw new Error('Template must be a string');
  }

  let result = template;

  for (const [key, value] of Object.entries(variables)) {
    if (!isString(value)) {
      console.warn(`Variable ${key} is not a string, converting...`);
      const stringValue = String(value);
      result = result.replace(new RegExp(`\\\\{\\\\{${key}\\\\}\\\\}`, 'g'), stringValue);
    } else {
      result = result.replace(new RegExp(`\\\\{\\\\{${key}\\\\}\\\\}`, 'g'), value);
    }
  }

  return result;
}

// Usage
const template = 'Hello {{name}}, you have {{count}} messages';
const variables = { name: 'John', count: '5' };
const result = processTemplate(template, variables);
console.log(result); // 'Hello John, you have 5 messages'

Playground​

import { isString } from '@winglet/common-utils';

// True cases - string type
console.log(isString('hello')); // true
console.log(isString('')); // true (empty string)
console.log(isString('123')); // true (numeric string)
console.log(isString('true')); // true (boolean string)
console.log(isString(`template string`)); // true (template literal)
console.log(isString(String(42))); // true (String constructor result)
console.log(isString('null')); // true (string containing 'null')

// False cases - not string type
console.log(isString(123)); // false (number)
console.log(isString(true)); // false (boolean)
console.log(isString(null)); // false
console.log(isString(undefined)); // false
console.log(isString(new String('hello'))); // false (String object, not primitive)
console.log(isString({})); // false (object)
console.log(isString([])); // false (array)

Notes​

Important Notes:

  • Only detects primitive string type, not String objects
  • Empty string ('') is a valid string and returns true
  • Template literals are treated as regular strings
  • String() constructor creates primitive strings when not used with new

String vs String Object:

  • 'hello' is a primitive string (returns true)
  • new String('hello') is a String object (returns false)
  • Use String(value) for type conversion without object creation

Use Cases:

  • Form field validation
  • API parameter validation
  • Text processing guards
  • Configuration validation
  • Array filtering
  • Template processing

Performance: Direct typeof check provides optimal performance.

Related Functions:

  • Use String() for type conversion
  • Use typeof value === 'string' for direct checking
  • Use isArray() for array detection
  • Use isNumber() for numeric validation