console.log(typeof 42);// Expected output: "number"console.log(typeof "blubber");// Expected output: "string"console.log(typeof true);// Expected output: "boolean"console.log(typeof undeclaredVariable);// Expected output: "undefined"

typeof operand

// Numberstypeof 37 === "number";typeof 3.14 === "number";typeof 42 === "number";typeof Math.LN2 === "number";typeof Infinity === "number";typeof NaN === "number"; // Despite being "Not-A-Number"typeof Number("1") === "number"; // Number tries to parse things into numberstypeof Number("shoe") === "number"; // including values that cannot be type coerced to a numbertypeof 42n === "bigint";// Stringstypeof "" === "string";typeof "bla" === "string";typeof `template literal` === "string";typeof "1" === "string"; // note that a number within a string is still typeof stringtypeof typeof 1 === "string"; // typeof always returns a stringtypeof String(1) === "string"; // String converts anything into a string, safer than toString// Booleanstypeof true === "boolean";typeof false === "boolean";typeof Boolean(1) === "boolean"; // Boolean() will convert values based on if they're truthy or falsytypeof !!1 === "boolean"; // two calls of the ! (logical NOT) operator are equivalent to Boolean()// Symbolstypeof Symbol() === "symbol";typeof Symbol("foo") === "symbol";typeof Symbol.iterator === "symbol";// Undefinedtypeof undefined === "undefined";typeof declaredButUndefinedVariable === "undefined";typeof undeclaredVariable === "undefined";// Objectstypeof { a: 1 } === "object";// use Array.isArray or Object.prototype.toString.call// to differentiate regular objects from arraystypeof [1, 2, 4] === "object";typeof new Date() === "object";typeof /regex/ === "object";// The following are confusing, dangerous, and wasteful. Avoid them.typeof new Boolean(true) === "object";typeof new Number(1) === "object";typeof new String("abc") === "object";// Functionstypeof function () {} === "function";typeof class C {} === "function";typeof Math.sin === "function";

// This stands since the beginning of JavaScripttypeof null === "object";

const str = new String("String");const num = new Number(100);typeof str; // "object"typeof num; // "object"const func = new Function();typeof func; // "function"

// Parentheses can be used for determining the data type of expressions.const someData = 99;typeof someData + " foo"; // "number foo"typeof (someData + " foo"); // "string"

typeof undeclaredVariable; // "undefined"

typeof newLetVariable; // ReferenceErrortypeof newConstVariable; // ReferenceErrortypeof newClass; // ReferenceErrorlet newLetVariable;const newConstVariable = "hello";class newClass {}

typeof document.all === "undefined";

function type(value) {  if (value === null) {    return "null";  }  const baseType = typeof value;  // Primitive types  if (!["object", "function"].includes(baseType)) {    return baseType;  }  // Symbol.toStringTag often specifies the "display name" of the  // object's class. It's used in Object.prototype.toString().  const tag = value[Symbol.toStringTag];  if (typeof tag === "string") {    return tag;  }  // If it's a function whose source code starts with the "class" keyword  if (    baseType === "function" &&    Function.prototype.toString.call(value).startsWith("class")  ) {    return "class";  }  // The name of the constructor; for example `Array`, `GeneratorFunction`,  // `Number`, `String`, `Boolean` or `MyCustomClass`  const className = value.constructor.name;  if (typeof className === "string" && className !== "") {    return className;  }  // At this point there's no robust way to get the type of value,  // so we use the base implementation.  return baseType;}

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