typcheckjs

Data type checks in Javascript.

Introduction

To check what data type something has in javascript is not always the easiest. The language itself provides an operator called typeof for that which works in a straightforward way. Typeof returns a string of what a values data type is, so for an object "object" is returned and for a string "string".

Installation

 npm install --save typcheckjs

Example

const Typcheckjs = require('typcheckjs');

// isString()
const isStringTrue = Typcheckjs.isString('hello');
// isStringTrue === true -> true

// isString()
const isStringFalse = Typcheckjs.isString(123);
// isStringFalse === true -> false

Usage

const Typcheckjs = require('typcheckjs');

// Returns true if a value is a string.
// A string is always a string so this one is easy.
// Except if called with new (new String) typeof will instead return "object".
// So to also include those strings instanceof can be used.
Typcheckjs.isString(value);

// Returns true if a value is really a number.
// From typeof more things than just an ordinary number will return "number" like NaN and Infinity. 
// To know if a value really is a number the function isFinite is also required.
Typcheckjs.isNumber(value);

// Returns true if a value is an array.
// In javascript arrays are not true arrays like in java and in other languages.
// They're actually objects so typeof will return "object" for them.
// To know if something's really an array its constructor can be compared to Array.
Typcheckjs.isArray(value);

// Returns true if a value is a function.
// Functions are functions so here just typeof is enough.
Typcheckjs.isFunction(value);

// Returns true if a value is an object.
// Many things are objects in javascript.
// To know if a value is an object that can have properties and be looped through,
// its constructor can be compared to Object. It doesn't work for objects created from classes,
// then the instanceof operator can be used instead.
Typcheckjs.isObject(value);

// Returns true if a value is null.
// Most times you don't need to check explicitly for null and
// undefined since they're both falsy values.
// However to do it below functions does the trick.
Typcheckjs.isNull(value);

// Returns true if a value is undefined.
// Most times you don't need to check explicitly for null and undefined since they're both falsy values.
Typcheckjs.isUndefined(value);

// Returns true if a value is a boolean.
// For booleans typeof is enough since it returns "boolean" for both true and false.
Typcheckjs.isBoolean(value);

// Returns true if a value is a regexp.
// RegExp's are objects so the only thing needed to check is if the constructor is RegExp.
Typcheckjs.isRegExp(value);

// Returns true if value is an error object.
// Errors in javascript are the same as "exceptions" in many other programming languages.
// They come in a couple different forms like for instance Error, TypeError and RangeError.
// An instanceof statement is enough for them all,
// but just to be extra sure we also check for the "message" property that errors have.
Typcheckjs.isError(value);

// Returns true if value is a date object.
// Date isn't really a data type in javascript.
// But to know if something's a Date object it can be checked with instanceof.
Typcheckjs.isDate(value);

// Returns true if a Symbol.
// In ES6 the new datatype Symbol was added.
// Nicely enough typeof returns "symbol" for it so no more logic is required.
Typcheckjs.isSymbol(value);

Tests

jay@ThinkPad  /usr/share/nginx/html/typcheck   master  npm test

> [email protected] test /usr/share/nginx/html/typcheck
> jest --coverage

 PASS  __tests__/index.test.js
  ✓ isString() should return true (4ms)
  ✓ isString() should return false
  ✓ isNumber() should return true (1ms)
  ✓ isNumber() should return false
  ✓ isArray() should return true
  ✓ isArray() should return false
  ✓ isFunction() should return true (1ms)
  ✓ isFunction() should return false
  ✓ isObject() should return true
  ✓ isObject() should return false
  ✓ isNull() should return true (1ms)
  ✓ isNull() should return false
  ✓ isUndefined() should return true
  ✓ isUndefined() should return false
  ✓ isBoolean() should return true (1ms)
  ✓ isBoolean() should return true
  ✓ isRegExp() should return true
  ✓ isRegExp() should return false
  ✓ isError() should return true (1ms)
  ✓ isError() should return false
  ✓ isDate() should return true
  ✓ isDate() should return false
  ✓ isSymbol() should return true (1ms)
  ✓ isSymbol() should return false

----------|----------|----------|----------|----------|-------------------|
File      |  % Stmts | % Branch |  % Funcs |  % Lines | Uncovered Line #s |
----------|----------|----------|----------|----------|-------------------|
All files |      100 |      100 |      100 |      100 |                   |
 index.js |      100 |      100 |      100 |      100 |                   |
----------|----------|----------|----------|----------|-------------------|
Test Suites: 1 passed, 1 total
Tests:       24 passed, 24 total
Snapshots:   0 total
Time:        1.276s
Ran all test suites.

Reference

https://webbjocke.com/javascript-check-data-types/