could-be-class

couldBeClass

Checks if an object could be an instantiable class.

This module also works with those classes that are written in ES5 style.

Example

"use strict";

const assert = require("assert");
const couldBeClass = require("could-be-class").default;

var str = "This is a string";
assert.ok(!couldBeClass(str));

var num = 123;
assert.ok(!couldBeClass(num));

class A { }
assert.ok(couldBeClass(A));

function B() {
    this.name = this.constructor.name;
}
assert.ok(couldBeClass(B));

function C() {
    this["name"] = this.constructor.name;
}
assert.ok(couldBeClass(C));

function D() { }
D.prototype.show = function () { }
assert.ok(couldBeClass(D));

function E() { }
Object.defineProperty(E.prototype, "size", {
    get() {
        return this.length;
    }
});
assert.ok(couldBeClass(E));

function TsEs5Class() {
    var _this = _super.call(this) || this;
    _this.desc = "This class form might be generated from TypeScript to ES5";
}
assert.ok(couldBeClass(TsEs5Class));

function notClass() {
    console.log("this is not a class");
}
assert.ok(!couldBeClass(notClass));

function IsClass() {
    console.log("this is a class");
}
assert.ok(couldBeClass(IsClass, true)); // strict mode

function aMethod() {
    this.desc = "this is a method instead of a class";
}
assert.ok(!couldBeClass(aMethod, true)); // strict mode

function* GenFunc() {
    this.desc = "Error will be thrown if trying to instantiate a Generator function.";
}
assert.ok(!couldBeClass(GenFunc));

if (parseFloat(process.version.slice(1)) >= 7.6) {
    // this block should only appear when you NodeJS version higher than 7.6, 
    // otherwise a SyntaxError will be thrown.
    async function AsyncFunc() {
        this.desc = "Error will be thrown if trying to instantiate an async function.";
    }
    assert.ok(!couldBeClass(AsyncFunc));
}

console.log("#### OK ####"); // should outout OK

Tip

Commonly, any functions (except async, generator and arrow functions) in JavaScript could be instantiated as a class constructor, however, in most cases, a class written in ES5 style will carry some special fingerprints that the detector can test if it could be a class constructor or just a pure function.

Strict Mode, you can pass the second argument strict: boolean to the function, if it is true, when testing ES5 class, will check if its name's first character is upper-cased, which is the common and recommended way to define a class. It always better to use the strict mode and code your classes in the recommended way.

WARN This module only performs about 98% percent of accuracy when testing ES5 class.