💾typeName()

JS ⟩ value ⟩ type ⟩ name ⟩ typeName()

Google Apps Script app helpers.
printPrototypeChain() - print the prototype chain of an object.

replit：typeName() (v.2)

typeof       base         type         expr         value       
--------------------------------------------------------------------------
object⭐️     Null         Null         null         null
--------------------------------------------------------------------------
undefined    Undefined    Undefined    undefined    undefined
--------------------------------------------------------------------------
number       Number       Number       37           37
number       Number       Number       3.14

replit：typeName() (v.2)
require： isClass(), baseTypeName()

// ⭐️ type name
// - require: 
//   • baseTypeName()
//   • isClass()
function typeName(value) {
    if (isClass(value)) return "class";    // class
    return baseTypeName(value);            // other cases
}

📁 module (copy and paste this file to use it)

// ⭐ base type name
function baseTypeName(value) {
    // --------------
    //          ↓   ↱ -1 (index of last character)
    //  0123456789012
    //          ╭──╮
    // "[object XXXX]"             ╭───╮
    //          ^^^^  <----  slice(8, -1)
    return Object.prototype.toString.call(value).slice(8, -1); 
}

/**    
 * ⭐️ content of function's definition
 *
 * @throws {TypeError} will throw a TypeError if value is not a function
 * @return {string}
 */
function functionDefinition(value) {
    
    // ---------------------------------------------------------------------
    // ⭐️ will throw a TypeError if value is not a function:
    //   ⛔ TypeError: 
    //      Function.prototype.toString requires that 'this' be a Function
    // ---------------------------------------------------------------------
    
    return Function.prototype.toString.call(value);
}

// ⭐️ check if value is a function
// - require: baseTypeName()
function isFunction(value) {
    return baseTypeName(value) === "Function";
}

// ⭐️ check if value is a "class"
// - require: 
//   • isFunction()
//   • functionDefinition()
function isClass(value) {
    return (
        // `value` is a function
        isFunction(value) &&
        // `value` is a "class" definition
        functionDefinition(value).startsWith("class")
    );
}

// ⭐️ type name
// - require: 
//   • baseTypeName()
//   • isClass()
function typeName(value) {
    if (isClass(value)) return "class";    // class
    return baseTypeName(value);            // other cases
}

// node exports
module.exports = { 
    baseTypeName,
    isFunction,
    functionDefinition,
    isClass,
    typeName, 
}

replit：typeName() (v.2)
📁 index.js

// ⭐ import (functions for type name)
const { typeName, baseTypeName } = require('./typeNames.js');
const { testCases } = require('./testCases.js');

// table settings
const header = ['typeof', 'base', 'type', 'expr', 'value'];
const cols = header.length;
const [width, pad, ext] = [12, 1, 10];
const line = '-'.repeat(width*cols + pad*(cols-1) + ext);

// header
console.log(...header.map(s => s.padEnd(width, ' ')));
console.log(line);

// rows
for (const testCase of testCases) {

    // separator
    if (testCase === '---') {
        console.log(line); 
        continue; 
    }
    
    // test cases
    const value = testCase[0];
    const expr = testCase[1] || String(testCase[0]);

    const types = [typeof value, baseTypeName(value), typeName(value)];
    const numberOfDifferentTypes = new Set(types.map(s => s.toLowerCase())).size;

    if (numberOfDifferentTypes > 1) types[0] += '⭐️';
    if (baseTypeName(value) !== typeName(value)) types[0] += '❗';
    
    console.log(
        ...[...types, expr].map(s => s.padEnd(width, ' ')),
        value,
    )
}

console.log(line);

// legend
console.log(`• ⭐️ types not all the same        • ❗ base !== type`);

📁 testCases.js

// custom class
class User {
    // ⭐ class field
    name = 'JohnDoe';
    // ⭐ custom class name
    get [Symbol.toStringTag]() { return 'User' }
}

// instance
let user = new User();

// test values: [expr, expr string]
const testCases = [  

    // ⭐️ null
    [null, ], 
    
    '---',

    // ⭐️ undefined --------------------------------------
    [undefined, ],

    '---',
    
    // ⭐️ Number --------------------------------------
    [37, ],
    [3.14, ],
    [Math.LN2, 'Math.LN2'],
    [Infinity, ],
    [NaN, ],
    [Number('1'), `Number('1')`],        // 1
    [Number('abc'), `Number('ab')`],     // NaN❗

    '---',

    // ⭐️ bigInt --------------------------------------
    [42n, ], 

    '---',

    // ⭐️ String --------------------------------------
    ['bla', `'bla'`],
    [`x = ${1+2}`, '`x = ${1+2}`'],   // 
    [typeof 1, 'typeof 1'],           // always returns a string
    [String({}), 'String({})'],       // [object]❗
    [typeof xxx, 'typeof xxx'],       // 'undefined'❗ (string)

    '---',

    // ⭐️ Boolean --------------------------------------
    [true, ],
    [Boolean(1), 'Boolean(1)'],    // true
    [!!(1), '!!(1)'],              // true

    '---',

    // ⭐️ Symbol --------------------------------------
    [Symbol(), ],
    [Symbol.iterator, 'Symbol.iterator'],

    '---',

    // ⭐️ objects --------------------------------------
    [{ a: 1 }, '{a:1}'],           // object literal
    [user, 'user'],                // instance of User
    
    [[1, 2], '[1, 2]'],            // Array
    [new Date(), 'new Date()'],    // Date
    [/regex/, ],                   // RegExp
    
    '---',

    // ⭐️ Functions --------------------------------------
    [function(){}, 'function(){}'],   // "normal" function
    [Math.sin, 'Math.sin'],           // method
    [() => {}, '() => {}'],           // arrow function
    
    [class {}, 'class {}'],           // class ⭐️
    [User, 'User'],                   // class ⭐️

    [function*(){}, 'function*(){}'], // generator functions

];

// export
module.exports = { User, testCases };

objects can change the behavior of Object.prototype.toString() by defining a Symbol.toStringTag property, leading to unexpected results:

const date = new Date();
date[Symbol.toStringTag] = 'XXX';
Date.prototype[Symbol.toStringTag] = 'prototype polluted';

const toString = Object.prototype.toString;    // keep it short
toString.call(date);           // [object XXX]
toString.call(new Date());     // [object prototype polluted]

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Last updated 2 years ago

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typeof base type expr value -------------------------------------------------------------------------- object⭐️ Null Null null null -------------------------------------------------------------------------- undefined Undefined Undefined undefined undefined -------------------------------------------------------------------------- number Number Number 37 37 number Number Number 3.14

// ⭐️ type name // - require: // • baseTypeName() // • isClass() function typeName(value) { if (isClass(value)) return "class"; // class return baseTypeName(value); // other cases }

// ⭐ base type name function baseTypeName(value) { // -------------- // ↓ ↱ -1 (index of last character) // 0123456789012 // ╭──╮ // "[object XXXX]" ╭───╮ // ^^^^ <---- slice(8, -1) return Object.prototype.toString.call(value).slice(8, -1); } /** * ⭐️ content of function's definition * * @throws {TypeError} will throw a TypeError if value is not a function * @return {string} */ function functionDefinition(value) { // --------------------------------------------------------------------- // ⭐️ will throw a TypeError if value is not a function: // ⛔ TypeError: // Function.prototype.toString requires that 'this' be a Function // --------------------------------------------------------------------- return Function.prototype.toString.call(value); } // ⭐️ check if value is a function // - require: baseTypeName() function isFunction(value) { return baseTypeName(value) === "Function"; } // ⭐️ check if value is a "class" // - require: // • isFunction() // • functionDefinition() function isClass(value) { return ( // `value` is a function isFunction(value) && // `value` is a "class" definition functionDefinition(value).startsWith("class") ); } // ⭐️ type name // - require: // • baseTypeName() // • isClass() function typeName(value) { if (isClass(value)) return "class"; // class return baseTypeName(value); // other cases } // node exports module.exports = { baseTypeName, isFunction, functionDefinition, isClass, typeName, }

// ⭐ import (functions for type name) const { typeName, baseTypeName } = require('./typeNames.js'); const { testCases } = require('./testCases.js'); // table settings const header = ['typeof', 'base', 'type', 'expr', 'value']; const cols = header.length; const [width, pad, ext] = [12, 1, 10]; const line = '-'.repeat(width*cols + pad*(cols-1) + ext); // header console.log(...header.map(s => s.padEnd(width, ' '))); console.log(line); // rows for (const testCase of testCases) { // separator if (testCase === '---') { console.log(line); continue; } // test cases const value = testCase[0]; const expr = testCase[1] || String(testCase[0]); const types = [typeof value, baseTypeName(value), typeName(value)]; const numberOfDifferentTypes = new Set(types.map(s => s.toLowerCase())).size; if (numberOfDifferentTypes > 1) types[0] += '⭐️'; if (baseTypeName(value) !== typeName(value)) types[0] += '❗'; console.log( ...[...types, expr].map(s => s.padEnd(width, ' ')), value, ) } console.log(line); // legend console.log(`• ⭐️ types not all the same • ❗ base !== type`);

// custom class class User { // ⭐ class field name = 'JohnDoe'; // ⭐ custom class name get [Symbol.toStringTag]() { return 'User' } } // instance let user = new User(); // test values: [expr, expr string] const testCases = [ // ⭐️ null [null, ], '---', // ⭐️ undefined -------------------------------------- [undefined, ], '---', // ⭐️ Number -------------------------------------- [37, ], [3.14, ], [Math.LN2, 'Math.LN2'], [Infinity, ], [NaN, ], [Number('1'), `Number('1')`], // 1 [Number('abc'), `Number('ab')`], // NaN❗ '---', // ⭐️ bigInt -------------------------------------- [42n, ], '---', // ⭐️ String -------------------------------------- ['bla', `'bla'`], [`x = ${1+2}`, '`x = ${1+2}`'], // [typeof 1, 'typeof 1'], // always returns a string [String({}), 'String({})'], // [object]❗ [typeof xxx, 'typeof xxx'], // 'undefined'❗ (string) '---', // ⭐️ Boolean -------------------------------------- [true, ], [Boolean(1), 'Boolean(1)'], // true [!!(1), '!!(1)'], // true '---', // ⭐️ Symbol -------------------------------------- [Symbol(), ], [Symbol.iterator, 'Symbol.iterator'], '---', // ⭐️ objects -------------------------------------- [{ a: 1 }, '{a:1}'], // object literal [user, 'user'], // instance of User [[1, 2], '[1, 2]'], // Array [new Date(), 'new Date()'], // Date [/regex/, ], // RegExp '---', // ⭐️ Functions -------------------------------------- [function(){}, 'function(){}'], // "normal" function [Math.sin, 'Math.sin'], // method [() => {}, '() => {}'], // arrow function [class {}, 'class {}'], // class ⭐️ [User, 'User'], // class ⭐️ [function*(){}, 'function*(){}'], // generator functions ]; // export module.exports = { User, testCases };

const date = new Date(); date[Symbol.toStringTag] = 'XXX'; Date.prototype[Symbol.toStringTag] = 'prototype polluted'; const toString = Object.prototype.toString; // keep it short toString.call(date); // [object XXX] toString.call(new Date()); // [object prototype polluted]