Object. The prototype. ToString. Call () and the difference between typeof and instanceof

1, the typeof

The typeof operator returns a string representing the typeof the unevaluated operand.

The sample

// Value typeof 37 ==='number'; Typeof = = = 3.14'number';
typeof(42) === 'number';
typeof Math.LN2 === 'number';
typeof Infinity === 'number';
typeof NaN === 'number'; // Although it is"Not-A-Number"Abbreviation typeof Number(1) ==='number'; // Number attempts to parse the argument to a numeric typeof 42n ==='bigint'; // String typeof' '= = ='string';
typeof 'bla'= = ='string';
typeof `template literal` === 'string';
typeof '1'= = ='string'; // Note that numeric strings are still the string typeof (typeof 1) ==='string'; // Typeof always returns a String typeof String(1) ==='string'; // String converts any value to a String, which is safer than toString // Boolean typeoftrue= = ='boolean';
typeof false= = ='boolean';
typeof Boolean(1) === 'boolean'; // Boolean() converts arguments based on whether they are true or imaginary typeof!! (1) = = ='boolean'; // Two calls! The (logical non) operator is equivalent to Boolean() // Symbols Typeof Symbol() ==='symbol';
typeof Symbol('foo') = = ='symbol';
typeof Symbol.iterator === 'symbol';


// Undefined
typeof undefined === 'undefined';
typeof declaredButUndefinedVariable === 'undefined';
typeof undeclaredVariable === 'undefined'; Typeof {a: 1} ==='object'; / / using Array. IsArray or Object. The prototype. ToString. Call / / distinguish between Array and ordinary objects typeof = = = [1, 2, 4]'object';

typeof new Date() === 'object';
typeof /regex/ === 'object'; // See the regular expressions section for historical results // The following examples are confusing, dangerous, and useless. Avoid them. typeof new Boolean(true) = = ='object';
typeof new Number(1) === 'object';
typeof new String('abc') = = ='object'; / / function typeoffunction() = = = {}'function';
typeof class C {} === 'function'
typeof Math.sin === 'function';Copy the code

The following table summarizes possible typeof return values.

typeofFor base types (
Undefined.
Null.
Boolean.
Number.
String.
BigInt.
Symbol
) in addition to
null
The correct type can be displayed for all objects, including arrays and objects
object
Is displayed for functions
function
. right
Array, object, timestamp, null, etc
object
type
, cannot accurately locate its type.

For arrays, objects, timestamps, we can use instance of to determine

2, instanceof

function Car(make, model, year) {
  this.make = make;
  this.model = model;
  this.year = year;
}
const auto = new Car('Honda'.'Accord', 1998);

console.log(auto instanceof Car);
// expected output: true

console.log(auto instanceof Object);
// expected output: trueCopy the code

The 👆 example shows that the constructor Car’s prototype property is on the prototype chain of the instance object Auto

Note: Instanceof can determine the data type, but its type is based on the Object.

Is there a better way to determine the type of variable?

Some can use Object. The prototype. ToString. Call (var), can more accurately judge the type of a variable.

3, the Object. The prototype. ToString. Call

Object. The prototype. ToString. Call () is in the Object. The prototype. The toString () method to implement the above, make its can determine a variety of data types