How do you check the difference between an ECMAScript 6 class and function?

Question

In ECMAScript 6 the typeof of classes is, according to the specification, \'function\'.

However also according to the specification you are

Answer 1

Ran some performance benchmarks on the different approaches mentioned in this thread, here is an overview:

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Native Class - Props Method (fastest by 56x on large examples, and 15x on trivial examples):

function isNativeClass (thing) {
    return typeof thing === 'function' && thing.hasOwnProperty('prototype') && !thing.hasOwnProperty('arguments')
}

Which works because the following is true:

> Object.getOwnPropertyNames(class A {})
[ 'length', 'name', 'prototype' ]
> Object.getOwnPropertyNames(class A { constructor (a,b) {} })
[ 'length', 'name', 'prototype' ]
> Object.getOwnPropertyNames(class A { constructor (a,b) {} a (b,c) {} })
[ 'length', 'name', 'prototype' ]
> Object.getOwnPropertyNames(function () {})
[ 'length', 'name', 'arguments', 'caller', 'prototype' ]
> Object.getOwnPropertyNames(() => {})
> [ 'length', 'name' ]

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Native Class - String Method (faster than regex method by about 10%):

/**
 * Is ES6+ class
 * @param {any} value
 * @returns {boolean}
 */
function isNativeClass (value /* :mixed */ ) /* :boolean */ {
    return typeof value === 'function' && value.toString().indexOf('class') === 0
}

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This may also be of use for determining a Conventional Class:

// Character positions
const INDEX_OF_FUNCTION_NAME = 9  // "function X", X is at index 9
const FIRST_UPPERCASE_INDEX_IN_ASCII = 65  // A is at index 65 in ASCII
const LAST_UPPERCASE_INDEX_IN_ASCII = 90   // Z is at index 90 in ASCII

/**
 * Is Conventional Class
 * Looks for function with capital first letter MyClass
 * First letter is the 9th character
 * If changed, isClass must also be updated
 * @param {any} value
 * @returns {boolean}
 */
function isConventionalClass (value /* :any */ ) /* :boolean */ {
    if ( typeof value !== 'function' )  return false
    const c = value.toString().charCodeAt(INDEX_OF_FUNCTION_NAME)
    return c >= FIRST_UPPERCASE_INDEX_IN_ASCII && c <= LAST_UPPERCASE_INDEX_IN_ASCII
}

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I'd also recommend checking out my typechecker package which includes the use cases for the above - via the isNativeClass method, isConventionalClass method, and a isClass method that checks for both types.