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Upcast is a low-level JavaScript type checking and casting library. Upcast simplifies type-checking and converts between types in a more sensible and predictable way than using plain ol' JavaScript.

Getting Started

You can use Upcast on the server side with Node.js and yarn/npm:

$ yarn add upcast
$ npm install upcast


Upcast exposes three simple functions:

  • type: get the type of an object
  • is: check whether an object is of a given type
  • to: convert an object to a specific type


Get the type of an object. This accepts a single argument: val: (mixed) The object to get the type of.

Types in Upcast are different to typeof in what is reported for arrays and null. See the example below:

upcast.type([]); // 'array'
upcast.type(true); // 'boolean'
upcast.type(function () {}); // 'function'
upcast.type(null); // 'null'
upcast.type(123); // 'number'
upcast.type({}); // 'object'
upcast.type('foo'); // 'string'
upcast.type(undefined); // 'undefined'

Check whether an object is of a given type. This accepts two arguments: val: (mixed) The object to check the type of. type: (string) The type to check for. One of array, boolean, function, null, number, object, string or undefined.

This function follows the same rules outlined in upcast.type and allows you to use type aliases.'foo', 'string'); // true, 'string'); // false[], 'array'); // true[], 'object'); // false, 'null'); // true, 'object'); // false

Convert an object to a specific type. This accepts two arguments: val: (mixed) The object to convert. type: (string) The type to convert to. One of array, boolean, function, null, number, object, string or undefined.

The way types are converted aims to be sensible and allow easy switching back-and-forth of common types. For example, switching between strings and arrays is quite fluid:'foo', 'array'); // ['f', 'o', 'o']['f', 'o', 'o'], 'string'); // 'foo'

You can use type aliases with this function. The examples below illustrate the way types are converted.

Converting to an array

Converting to an array from a boolean, function, number or object simply wraps the value in an array:, 'array'); // [123]

Strings are handled differently, an array is returned with each character in the string as an item:'foo', 'array'); // ['f', 'o', 'o']

Null and undefined are converted to an empty array:, 'array'); // []

Converting to a boolean

Boolean conversion simply converts to true or false based on whether the value is truthy or not. The only case where this doesn't follow JavaScript's standard behaviour is with empty arrays which are converted to false:[1, 2, 3], 'boolean') // true[], 'boolean') // false

Converting to a function

When converting to a function, the original value is simply wrapped in a new function. This function returns the original value:'foo', 'function'); // function () { return 'foo'; }

Converting to null

As expected, converting to null will always return null:'foo', 'null'); // null

Converting to a number

Converting to a number from a boolean, function, null or object simply calls Number with the original value as an argument, returning the expected value:'true', 'number'); // 1

Arrays and strings are handled differently, an array is joined to create a string, then evaluated with parseInt; strings are simply evaluated with parseInt:[1, 2, 3], 'number'); // 123'123', 'number'); // 123'foo', 'number'); // 0

Undefined is converted to 0 rather than NaN:, 'number'); // 0

Converting to an object

Converting to an object simply calls Object with the value as a first argument. The following are equivalent:'foo', 'object');

Converting to a string

Converting to a string from a boolean, function, number or object simply returns the value added to an empty string, using JavaScript's default type conversion:, 'string'); // 'true', 'string'); // '123'

Arrays are handled differently, they are joined with an empty string:['f', 'o', 'o'], 'string'); // 'foo'

Null and undefined are converted to an empty string rather than 'null' and 'undefined':, 'string'); // ''

Converting to undefined

As expected, converting to undefined will always return undefined:'foo', 'undefined'); // undefined

Type aliases

The is and to functions allow you to use aliases to certain core types. The following are equivalent:[], 'array');[], 'arr');[], 'a');

The aliases available by default are:

  • array: arr, a
  • boolean: bool, b
  • function: fn, f
  • number: num, n
  • object: obj, o
  • string: str, s


To work on Upcast you'll need to clone the repo and it's install dependencies with git clone && cd upcast && yarn install.

Once you're set up, you can run the following commands:

$ yarn lint           # Run xo to lint all js files
$ yarn test           # Run unit tests with ava
$ yarn test-coverage  # Run unit tests and coverage report with ava + nyc


Upcast is licensed under the MIT license.