How to Implement the instanceof in Javascript?

Time：2020-09-17 14:37:27
Class：Weblog
Read：37

NodeJs / Javascript

In Javascript, the operator instanceof is used to perform a test to check if the prototype property of a constructor apperas in the chain of the object’s prototype chain.

The syntax to use is:

1
2
3

object instanceof constructor
// object - the object to test
// constructor - to test against

object instanceof constructor
// object - the object to test
// constructor - to test against

For example:

function Car() {}
function Train() {}
 
const car = new Car();
const train = new Train();
 
console.log(car instanceof Car); // true
console.log(car instanceof Train); // false
console.log(train instanceof Car); // false;
console.log(train instanceof Train); // true;

function Car() {}
function Train() {}

const car = new Car();
const train = new Train();

console.log(car instanceof Car); // true
console.log(car instanceof Train); // false
console.log(train instanceof Car); // false;
console.log(train instanceof Train); // true;

We can implement a instanceOf function in pure Javascript that does the same instanceof check – probably useful in front-end Javascript interview:

function instanceOf(left, right) {
    let proto = left.__proto__;
    let prototype = right.prototype;
    for (;;) {
        if (proto === null) return false;
        if (proto === prototype) return true;
        proto = proto.__proto__; // trace way up along the chain
    }
}

function instanceOf(left, right) {
    let proto = left.__proto__;
    let prototype = right.prototype;
    for (;;) {
        if (proto === null) return false;
        if (proto === prototype) return true;
        proto = proto.__proto__; // trace way up along the chain
    }
}

And it expects to produce the same results:

console.log(instanceOf(car, Car)); // true
console.log(instanceOf(car, Train)); // false
console.log(instanceOf(train, Car)); // false;
console.log(instanceOf(train, Train)); // true;

console.log(instanceOf(car, Car)); // true
console.log(instanceOf(car, Train)); // false
console.log(instanceOf(train, Car)); // false;
console.log(instanceOf(train, Train)); // true;

The idea is conduct a loop, and trace up the prototype property along the chain until we have a match (equal prototype in the chain) or we have reached the end of the chain – which is null.

–EOF (The Ultimate Computing & Technology Blog) —

Recommend：
Minimum Numbers of Function Calls to Make Target Array
Harddrives will fail – it is just a matter of when
Bruteforce/DFS/Backtracking Algorithm using Javascript to Solve
DFS and BFS Algorithm to Find Numbers With Same Consecutive Diff
How to Remove Items/Entries with Specific Values from Map/HashMa
Find the Real Root of 4^x + 6^x = 9^x
Depth First Search (Backtracking) Algorithm to Solve a Sudoku Ga
Using Bitmasking Algorithm to Compute the Combinations of an Arr
Flashing the BIOS of HPZ800 Server to 3.61 Rev.A
Algorithm to Sum The Fibonacci Numbers

Comment list

Comment add: Name：　Check：